Methods and apparatus for ventilatory treatment of respiratory disorders

ABSTRACT

Disclosed is an apparatus for treating a respiratory disorder in a patient. The apparatus comprises a pressure generator configured to deliver a flow of air at positive pressure to an airway of the patient through a patient interface, a sensor configured to generate a signal representative of respiratory flow rate of the patient, and a controller. The controller is configured to control the pressure generator to deliver ventilation therapy having a base pressure and a pressure support through the patient interface, detect an apnea from the signal representative of respiratory flow rate of the patient, control the pressure generator to deliver one or more probe breaths to the patient during the apnea, determine patency of the patient&#39;s airway from a waveform of the respiratory flow rate signal in response to one of the one or more probe breaths, compute an effective duration of the apnea based on the patency of the airway, and adjust a set point for the base pressure of the ventilation therapy in response to the apnea based on the effective duration of the apnea.

1 CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a national phase entry under 35 U.S.C. § 371of International Application No. PCT/AU2017/051265 filed Nov. 17, 2017,published in English, which claims priority from Australian ProvisionalPatent Application No. 2016904724 filed Nov. 18, 2016, all of which areincorporated herein by reference.

2 STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

3 SEQUENCE LISTING

Not Applicable

4 BACKGROUND OF THE TECHNOLOGY 4.1 Field of the Technology

The present technology relates to one or more of the detection,diagnosis, treatment, prevention and amelioration of respiratory-relateddisorders. The present technology also relates to medical devices orapparatus, and their use.

4.2 Description of the Related Art

4.2.1 Human Respiratory System and its Disorders

The respiratory system of the body facilitates gas exchange. The noseand mouth form the entrance to the airways of a patient.

The airways include a series of branching tubes, which become narrower,shorter and more numerous as they penetrate deeper into the lung. Theprime function of the lung is gas exchange, allowing oxygen to move fromthe air into the venous blood and carbon dioxide to move out. Thetrachea divides into right and left main bronchi, which further divideeventually into terminal bronchioles. The bronchi make up the conductingairways, and do not take part in gas exchange. Further divisions of theairways lead to the respiratory bronchioles, and eventually to thealveoli. The alveolated region of the lung is where the gas exchangetakes place, and is referred to as the respiratory zone. See“Respiratory Physiology”, by John B. West, Lippincott Williams &Wilkins, 9th edition published 2011.

A range of respiratory disorders exist. Certain disorders may becharacterised by particular events, e.g. apneas, hypopneas, andhyperpneas.

Obstructive Sleep Apnea (OSA), a form of Sleep Disordered Breathing(SDB), is characterized by events including occlusion or obstruction ofthe upper air passage during sleep. It results from a combination of anabnormally small upper airway and the normal loss of muscle tone in theregion of the tongue, soft palate and posterior oropharyngeal wallduring sleep. The condition causes the affected patient to stopbreathing for periods typically of 30 to 120 seconds in duration,sometimes 200 to 300 times per night. It often causes excessive daytimesomnolence, and it may cause cardiovascular disease and brain damage.The syndrome is a common disorder, particularly in middle agedoverweight males, although a person affected may have no awareness ofthe problem. See U.S. Pat. No. 4,944,310 (Sullivan).

Cheyne-Stokes Respiration (CSR) is another form of sleep disorderedbreathing. CSR is a disorder of a patient's respiratory controller inwhich there are rhythmic alternating periods of waxing and waningventilation known as CSR cycles. CSR is characterised by repetitivede-oxygenation and re-oxygenation of the arterial blood. It is possiblethat CSR is harmful because of the repetitive hypoxia. In some patientsCSR is associated with repetitive arousal from sleep, which causessevere sleep disruption, increased sympathetic activity, and increasedafterload. See U.S. Pat. No. 6,532,959 (Berthon-Jones).

Respiratory Failure is an umbrella term for respiratory disorders inwhich patients are unable to ventilate enough to balance the CO₂ intheir blood if their metabolic activity rises much above rest.Respiratory failure encompasses all of the following conditions.

Obesity Hyperventilation Syndrome (OHS) is defined as the combination ofsevere obesity and awake chronic hypercapnia, in the absence of otherknown causes for hypoventilation. Symptoms include dyspnea, morningheadache and excessive daytime sleepiness.

Chronic Obstructive Pulmonary Disease (COPD) encompasses any of a groupof lower airway diseases that have certain characteristics in common.These include increased resistance to air movement, extended expiratoryphase of respiration, and loss of the normal elasticity of the lung.Examples of COPD are emphysema and chronic bronchitis. COPD is caused bychronic tobacco smoking (primary risk factor), occupational exposures,air pollution and genetic factors. Symptoms include: dyspnea onexertion, chronic cough and sputum production.

Neuromuscular Disease (NMD) is a broad term that encompasses manydiseases and ailments that impair the functioning of the muscles eitherdirectly via intrinsic muscle pathology, or indirectly via nervepathology. Some NMD patients are characterised by progressive muscularimpairment leading to loss of ambulation, being wheelchair-bound,swallowing difficulties, respiratory muscle weakness and, eventually,death from respiratory failure. Neuromuscular disorders can be dividedinto rapidly progressive and slowly progressive: (i) Rapidly progressivedisorders: Characterised by muscle impairment that worsens over monthsand results in death within a few years (e.g. Amyotrophic lateralsclerosis (ALS) and Duchenne muscular dystrophy (DMD) in teenagers);(ii) Variable or slowly progressive disorders: Characterised by muscleimpairment that worsens over years and only mildly reduces lifeexpectancy (e.g. Limb girdle, Facioscapulohumeral and Myotonic musculardystrophy). Symptoms of respiratory failure in NMD include: increasinggeneralised weakness, dysphagia, dyspnea on exertion and at rest,fatigue, sleepiness, morning headache, and difficulties withconcentration and mood changes.

Chest wall disorders are a group of thoracic deformities that result ininefficient coupling between the respiratory muscles and the thoraciccage. The disorders are usually characterised by a restrictive defectand share the potential of long term hypercapnic respiratory failure.Scoliosis and/or kyphoscoliosis may cause severe respiratory failure.Symptoms of respiratory failure include: dyspnea on exertion, peripheraloedema, orthopnea, repeated chest infections, morning headaches,fatigue, poor sleep quality and loss of appetite.

A range of therapies have been used to treat or ameliorate suchconditions. Furthermore, otherwise healthy individuals may takeadvantage of such therapies to prevent respiratory disorders fromarising. However, these have a number of shortcomings.

4.2.2 Therapies

Continuous Positive Airway Pressure (CPAP) therapy has been used totreat Obstructive Sleep Apnea (OSA). The mechanism of action is thatcontinuous positive airway pressure acts as a pneumatic splint and mayprevent upper airway occlusion, such as by pushing the soft palate andtongue forward and away from the posterior oropharyngeal wall. Treatmentof OSA by CPAP therapy may be voluntary, and hence patients may electnot to comply with therapy if they find devices used to provide suchtherapy one or more of: uncomfortable, difficult to use, expensive andaesthetically unappealing.

Non-invasive ventilation (NIV) provides ventilatory support to a patientthrough the upper airways to assist the patient breathing and/ormaintain adequate oxygen levels in the body by doing some or all of thework of breathing. The ventilatory support is provided via anon-invasive patient interface. NIV has been used to treat CSR andrespiratory failure, in forms such as OHS, COPD, NMD, and Chest Walldisorders. In some forms, the comfort and effectiveness of thesetherapies may be improved.

Patients receiving non-invasive ventilation, particularly when asleepand/or under sedation, are often subject to upper airway instability andcollapse, as in OSA. Such instability and collapse can compromise theeffectiveness of the ventilation therapy by reducing or even nullifyingthe pressure actually reaching the lungs from the ventilator.

The upper airway can be stabilised by maintaining a positive basepressure, referred to herein as the EPAP, upon which ventilatoryassistance is superimposed. An insufficient EPAP permits upper airwaycollapse, while an excessive EPAP may fully stabilise the upper airwaybut negatively impact on comfort, promote mask leak, or posecardiovascular complications. The task of choosing an EPAP that issufficient to generally maintain upper airway stability across the rangeof sleep states, posture, level of sedation, and progression of diseasewhile avoiding negative side-effects (a task known as EPAP titration) isa significant challenge even for experienced clinicians with the benefitof a full polysomnographic (PSG) study. An appropriately titrated EPAPis a balance between extremes, not necessarily one that prevents allobstructive events. While NIV enjoys growing usage globally, only afraction of patients are administered NIV with the benefit of a PSGstudy to titrate the EPAP. In more acute environments, historicallythere is limited awareness of the effects of sleep and sedation on theefficacy of non-invasive ventilation.

There is therefore a significant need for NIV therapies capable ofautomatically adjusting the EPAP (i.e. performing “EPAP auto-titration”)in dynamic response to the changing condition of an NIV patient's upperairway.

4.2.3 Treatment Systems

These therapies may be provided by a treatment system or device. Suchsystems and devices may also be used to diagnose a condition withouttreating it.

A treatment system may comprise a Respiratory Pressure Therapy Device(RPT device), an air circuit, a humidifier, a patient interface, anddata management.

4.2.3.1 Patient Interface

A patient interface may be used to interface respiratory equipment toits wearer, for example by providing a flow of air to an entrance to theairways. The flow of air may be provided via a mask to the nose and/ormouth, a tube to the mouth or a tracheostomy tube to the trachea of apatient. Depending upon the therapy to be applied, the patient interfacemay form a seal, e.g., with a region of the patient's face, tofacilitate the delivery of gas at a pressure at sufficient variance withambient pressure to effect therapy, e.g., at a positive pressure ofabout 10 cmH₂O relative to ambient pressure. For other forms of therapy,such as the delivery of oxygen, the patient interface may not include aseal sufficient to facilitate delivery to the airways of a supply of gasat a positive pressure of about 10 cmH₂O.

4.2.3.2 Respiratory Pressure Therapy (RPT) Device

Air pressure generators are known in a range of applications, e.g.industrial-scale ventilation systems. However, air pressure generatorsfor medical applications have particular requirements not fulfilled bymore generalised air pressure generators, such as the reliability, sizeand weight requirements of medical devices. In addition, even devicesdesigned for medical treatment may suffer from shortcomings, pertainingto one or more of: comfort, noise, ease of use, efficacy, size, weight,manufacturability, cost, and reliability.

One known RPT device used for treating sleep disordered breathing is theS9 Sleep Therapy System, manufactured by ResMed Limited. Another exampleof an RPT device is a ventilator. Ventilators such as the ResMedStellar™ Series of Adult and Paediatric Ventilators may provide supportfor invasive and non-invasive non-dependent ventilation for a range ofpatients for treating a number of conditions such as but not limited toNMD, OHS and COPD.

The ResMed Elisée™ 150 ventilator and ResMed VS III™ ventilator mayprovide support for invasive and non-invasive dependent ventilationsuitable for adult or paediatric patients for treating a number ofconditions. These ventilators provide volumetric and barometricventilation modes with a single or double limb circuit. RPT devicestypically comprise a pressure generator, such as a motor-driven bloweror a compressed gas reservoir, and are configured to supply a flow ofair to the airway of a patient. In some cases, the flow of air may besupplied to the airway of the patient at positive pressure. The outletof the RPT device is connected via an air circuit to a patient interfacesuch as those described above.

4.2.3.3 Humidifier

Delivery of a flow of air without humidification may cause drying ofairways. The use of a humidifier with an RPT device and the patientinterface produces humidified gas that minimizes drying of the nasalmucosa and increases patient airway comfort. In addition in coolerclimates, warm air applied generally to the face area in and about thepatient interface is more comfortable than cold air. A range ofartificial humidification devices and systems are known, however theymay not fulfil the specialised requirements of a medical humidifier.

5 BRIEF SUMMARY OF THE TECHNOLOGY

The present technology is directed towards providing medical devicesused in the diagnosis, amelioration, treatment, or prevention ofrespiratory disorders having one or more of improved comfort, cost,efficacy, ease of use and manufacturability.

A first aspect of the present technology relates to apparatus used inthe diagnosis, amelioration, treatment or prevention of a respiratorydisorder.

Another aspect of the present technology relates to methods used in thediagnosis, amelioration, treatment or prevention of a respiratorydisorder.

The present technology comprises methods and apparatus for ventilatorytherapy for respiratory disorders that automatically titrate a basepressure of the ventilation to maintain upper airway stability such thatthe applied ventilatory assistance can reach the lungs of the patient.The auto-titration increases the base pressure by an amount generallyproportional to the severity of detected apneas and/or episodes of flowlimitation, and decreases the base pressure toward a minimum value inthe absence of such episodes. In particular, the adjustment takes intoaccount the open/closed status of the airway during the apnea, asdetermined by analysis of the flow response to timed backup breathsdelivered during the apnea.

According to one form of the present technology, there is provided anapparatus for treating a respiratory disorder in a patient. Theapparatus comprises a pressure generator configured to deliver a flow ofair at positive pressure to an airway of the patient through a patientinterface, a sensor configured to generate a signal representative ofrespiratory flow rate of the patient, and a controller. The controlleris configured to control the pressure generator to deliver ventilationtherapy having a base pressure and a pressure support through thepatient interface, detect an apnea from the signal representative ofrespiratory flow rate of the patient, control the pressure generator todeliver one or more probe breaths to the patient during the apnea,determine patency of the patient's airway from a waveform of therespiratory flow rate signal in response to one of the one or more probebreaths, compute an effective duration of the apnea based on the patencyof the airway, and adjust a set point for the base pressure of theventilation therapy in response to the apnea based on the effectiveduration of the apnea.

According to another form of the present technology, there is provided amethod of treating a respiratory disorder in a patient. The methodcomprises controlling a pressure generator to deliver a ventilationtherapy through a patient interface to the patient, the ventilationtherapy having a base pressure and a pressure support; detecting, in acontroller of the pressure generator, an apnea from a signalrepresentative of respiratory flow rate of the patient; controlling thepressure generator to deliver one or more probe breaths to the patientduring the apnea; determining patency of the patient's airway from awaveform of the respiratory flow rate signal in response to one of theone or more probe breaths; computing an effective duration of the apneabased on the patency of the airway; and adjusting a set point for thebase pressure of the ventilation therapy in response to the apnea basedon the effective duration of the apnea.

According to yet another form of the present technology, there isprovided a respiratory therapy system comprising: means for delivering aventilation therapy through a patient interface to a patient, theventilation therapy having a base pressure and a pressure support; meansfor generating a signal representative of respiratory flow rate of thepatient; means for detecting an apnea from a the respiratory flow ratesignal; means for delivering one or more probe breaths to the patientduring the apnea; means for determining patency of the patient's airwayfrom a waveform of the respiratory flow rate signal in response to oneof the one or more probe breaths; means for computing an effectiveduration of the apnea based on the patency of the airway; and means foradjusting a set point for the base pressure of the ventilation therapyin response to the apnea based on the effective duration of the apnea.

According to yet another form of the present technology, there isprovided a method of determining patency of the airway during an apneaof a patient. The method comprises: controlling a pressure generator todeliver one or more probe breaths to the patient during the apnea; anddetermining patency of the airway from a signal representative ofrespiratory flow rate of the patient during the apnea, wherein thedetermining is dependent on the shape of a waveform of the respiratoryflow rate signal in response to at least one of the one or more probebreaths.

According to yet another form of the present technology, there isprovided an apparatus for treating a respiratory disorder in a patient.The apparatus comprises: a pressure generator configured to deliver aflow of air at positive pressure to an airway of the patient through apatient interface; a sensor configured to generate a signalrepresentative of respiratory flow rate of the patient; and a controllerconfigured to: control the pressure generator to deliver ventilationtherapy through the patient interface; control the pressure generator todeliver one or more probe breaths to the patient during an apnea of thepatient; and determine patency of the airway from a waveform of therespiratory flow rate signal during the apnea, wherein determining thepatency is dependent on the shape of the respiratory flow rate waveformin response to at least one of the one or more probe breaths.

According to yet another form of the present technology, there isprovided a respiratory therapy system comprising: means for delivering aflow of air at positive pressure to an airway of a patient through apatient interface; means for generating a signal representative ofrespiratory flow rate of the patient; means for delivering one or moreprobe breaths to the patient during an apnea of the patient; and meansfor determining patency of the airway from a waveform of the respiratoryflow rate signal during the apnea. The determining patency is dependenton the shape of the respiratory flow rate waveform in response to atleast one of the probe breaths.

Of course, portions of the aspects may form sub-aspects of the presenttechnology. Also, various ones of the sub-aspects and/or aspects may becombined in various manners and also constitute additional aspects orsub-aspects of the present technology.

Other features of the technology will be apparent from consideration ofthe information contained in the following detailed description,abstract, drawings and claims.

6 BRIEF DESCRIPTION OF THE DRAWINGS

The present technology is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings, in whichlike reference numerals refer to similar elements including:

6.1 Treatment Systems

FIG. 1 shows a system including a patient 1000 wearing a patientinterface 3000, in the form of a full-face mask, receiving a supply ofair at positive pressure from an RPT device 4000. Air from the RPTdevice is humidified in a humidifier 5000, and passes along an aircircuit 4170 to the patient 1000.

6.2 Respiratory System and Facial Anatomy

FIG. 2 shows an overview of a human respiratory system including thenasal and oral cavities, the larynx, vocal folds, oesophagus, trachea,bronchus, lung, alveolar sacs, heart and diaphragm.

6.3 Patient Interface

FIG. 3 shows a patient interface in the form of a nasal mask inaccordance with one form of the present technology.

6.4 RPT Device

FIG. 4A shows an RPT device in accordance with one form of the presenttechnology.

FIG. 4B is a schematic diagram of the pneumatic path of an RPT device inaccordance with one form of the present technology. The directions ofupstream and downstream are indicated.

FIG. 4C is a schematic diagram of the electrical components of an RPTdevice in accordance with one form of the present technology.

FIG. 4D is a schematic diagram of the algorithms implemented in an RPTdevice in accordance with one form of the present technology.

6.5 Humidifier

FIG. 5A shows an isometric view of a humidifier in accordance with oneform of the present technology.

FIG. 5B shows an isometric view of a humidifier in accordance with oneform of the present technology, showing a humidifier reservoir 5110removed from the humidifier reservoir dock 5130.

6.6 Breathing Waveforms

FIG. 6A shows a typical model respiratory flow rate waveform of a personwhile sleeping. The horizontal axis is time, and the vertical axis isrespiratory flow rate. While the parameter values may vary, a typicalbreath may have the following approximate values: tidal volume, Vt, 0.5L, inspiratory time, Ti, 1.6 s, peak inspiratory flow rate, Qpeak, 0.4L/s, expiratory time, Te, 2.4 s, peak expiratory flow rate, Qpeak, −0.5L/s. The total duration of the breath, Ttot, is about 4 s. The persontypically breathes at a rate of about 15 breaths per minute (BPM), withVentilation, Vent, about 7.5 L/minute. A typical duty cycle, the ratioof Ti to Ttot is about 40%.

FIG. 6B shows a scaled inspiratory portion of a respiratory flow ratewaveform where the patient is experiencing an example of “classicalflatness” inspiratory flow limitation.

FIG. 6C shows a scaled inspiratory portion of a respiratory flow ratewaveform where the patient is experiencing an example of “chair-shaped”(late flatness) inspiratory flow limitation.

FIG. 6D shows a scaled inspiratory portion of a respiratory flow ratewaveform where the patient is experiencing an example of “reverse chair”(early flatness) inspiratory flow limitation.

FIG. 6E shows a scaled inspiratory portion of a respiratory flow ratewaveform where the patient is experiencing an example of “M-shaped”inspiratory flow limitation.

FIG. 6F shows a scaled inspiratory portion of a respiratory flow ratewaveform where the patient is experiencing an example of severely“M-shaped” inspiratory flow limitation.

6.7 EPAP Auto-Titration

FIG. 7A is a flow chart illustrating a method that may be used toimplement the inspiratory flow limitation determination algorithm ofFIG. 4D.

FIG. 7B is a flow chart illustrating a method that may be used toimplement the central part feature computation step of FIG. 7A.

FIG. 7C is a flow chart illustrating a method that may be used toimplement the flow limitation fuzzy truth variable computation step ofFIG. 7A.

FIG. 7D is a flow chart illustrating a method that may be used toimplement one of the flow limitation fuzzy truth variable computationsteps of FIG. 7C.

FIG. 7E is a flow chart illustrating a method that may be used toimplement the late flatness fuzzy truth variable computation step ofFIG. 7A.

FIG. 7F is a flow chart illustrating a method that may be used toimplement the M-shape detection algorithm of FIG. 4D.

FIG. 7G is a flow chart illustrating a method that may be used toimplement the apnea detection algorithm of FIG. 4D.

FIGS. 7H and 7I is a flow chart illustrating a method that may be usedto implement the airway patency determination algorithm of FIG. 4D.

FIG. 8A is a flow chart illustrating a method of auto-titrating the EPAPvalue that may be implemented by the therapy parameter determinationalgorithm of FIG. 4D.

FIG. 8B is a flow chart illustrating a method that may be used toimplement the “Shape Doctor” step of the EPAP-auto-titration method ofFIG. 8A.

FIG. 8C is a flow chart illustrating a method that may be used toimplement one step of the method of FIG. 8B.

FIG. 8D is a flow chart illustrating a method that may be used toimplement another step of the method of FIG. 8B.

FIG. 8E is a flow chart illustrating a method that may be used toimplement the “Apnea Doctor” step of the EPAP-auto-titration method ofFIG. 8A.

FIG. 8F is a flow chart illustrating a method that may be used toimplement the apnea processing step of the method of FIG. 8E.

FIG. 8G is a flow chart illustrating a method that may be used toimplement a step of the EPAP-auto-titration method of FIG. 8A.

FIG. 9A contains a graph illustrating an example of the behaviour of theEPAP auto-titration method of FIG. 8A in response to episodes of flowlimitation.

FIG. 9B contains a graph illustrating an example of the behaviour of theEPAP auto-titration method of FIG. 8A in response to a closed apnea.

FIG. 9C contains a graph illustrating an example of the behaviour of theairway patency determination algorithm of FIG. 7H in response to a mixedapnea.

FIG. 9D contains expansions of various sections of the graph of FIG. 9C.

7 DETAILED DESCRIPTION OF EXAMPLES OF THE TECHNOLOGY

Before the present technology is described in further detail, it is tobe understood that the technology is not limited to the particularexamples described herein, which may vary. It is also to be understoodthat the terminology used in this disclosure is for the purpose ofdescribing only the particular examples discussed herein, and is notintended to be limiting.

The following description is provided in relation to various exampleswhich may share one or more common characteristics and/or features. Itis to be understood that one or more features of any one example may becombinable with one or more features of another example or otherexamples. In addition, any single feature or combination of features inany of the examples may constitute a further example.

7.1 Therapy

In one form, the present technology comprises a method for treating arespiratory disorder comprising the step of delivering air at positivepressure to the entrance of the airways of a patient 1000.

In certain examples of the present technology, a supply of air atpositive pressure is provided to the nasal passages of the patient viaone or both nares.

7.2 Treatment Systems

In one form, the present technology comprises an apparatus or device fortreating a respiratory disorder. The apparatus or device may comprise anRPT device 4000 for delivering pressurised air to the patient 1000 viaan air circuit 4170 to a patient interface 3000.

7.3 Patient Interface

A non-invasive patient interface 3000 in accordance with one aspect ofthe present technology comprises the following functional aspects: aseal-forming structure 3100, a plenum chamber 3200, a positioning andstabilising structure 3300, a vent 3400, one form of connection port3600 for connection to air circuit 4170, and a forehead support 3700. Insome forms a functional aspect may be provided by one or more physicalcomponents. In some forms, one physical component may provide one ormore functional aspects. In use the seal-forming structure 3100 isarranged to surround an entrance to the airways of the patient so as tofacilitate the delivery of air at positive pressure to the airways.

7.4 RPT Device

An RPT device 4000 in accordance with one aspect of the presenttechnology comprises mechanical and pneumatic components 4100,electrical components 4200 and is configured to execute one or morealgorithms 4300. The RPT device may have an external housing 4010,formed in two parts, an upper portion 4012 and a lower portion 4014.Furthermore, the external housing 4010 may include one or more panel(s)4015. The RPT device 4000 comprises a chassis 4016 that supports one ormore internal components of the RPT device 4000. The RPT device 4000 mayinclude a handle 4018.

The pneumatic path of the RPT device 4000 may comprise one or more airpath items, e.g., an inlet air filter 4112, an inlet muffler 4122, apressure generator 4140 capable of delivering air at positive pressure(e.g., a blower 4142), an outlet muffler 4124 and one or moretransducers 4270, such as pressure sensors 4272 and flow rate sensors4274.

One or more of the air path items may be located within a removableunitary structure which will be referred to as a pneumatic block 4020.The pneumatic block 4020 may be located within the external housing4010. In one form a pneumatic block 4020 is supported by, or formed aspart of the chassis 4016.

The RPT device 4000 may have an electrical power supply 4210, one ormore input devices 4220, a central controller 4230, a therapy devicecontroller 4240, a pressure generator 4140, one or more protectioncircuits 4250, memory 4260, transducers 4270, data communicationinterface 4280 and one or more output devices 4290. Electricalcomponents 4200 may be mounted on a single Printed Circuit BoardAssembly (PCBA) 4202. In an alternative form, the RPT device 4000 mayinclude more than one PCBA 4202.

7.4.1 RPT Device Mechanical & Pneumatic Components

An RPT device may comprise one or more of the following components in anintegral unit. In an alternative form, one or more of the followingcomponents may be located as respective separate units.

7.4.1.1 Air Filter(s)

An RPT device in accordance with one form of the present technology mayinclude an air filter 4110, or a plurality of air filters 4110.

In one form, an inlet air filter 4112 is located at the beginning of thepneumatic path upstream of a pressure generator 4140.

In one form, an outlet air filter 4114, for example an antibacterialfilter, is located between an outlet of the pneumatic block 4020 and apatient interface 3000.

7.4.1.2 Muffler(s)

In one form of the present technology, an inlet muffler 4122 is locatedin the pneumatic path upstream of a pressure generator 4140.

In one form of the present technology, an outlet muffler 4124 is locatedin the pneumatic path between the pressure generator 4140 and a patientinterface 3000.

7.4.1.3 Pressure Generator

In one form of the present technology, a pressure generator 4140 fordelivering a flow, or a supply, of air at positive pressure is acontrollable blower 4142. For example the blower 4142 may include abrushless DC motor 4144 with one or more impellers housed in a volute.The blower may be capable of delivering a supply of air, for example ata rate of up to about 120 litres/minute, at a positive pressure in arange from about 4 cmH₂O to about 20 cmH₂O, or in other forms up toabout 30 cmH₂O. The blower may be as described in any one of thefollowing patents or patent applications the contents of which areincorporated herein by reference in their entirety: U.S. Pat. Nos.7,866,944; 8,638,014; 8,636,479; and PCT Patent Application PublicationNo. WO 2013/020167.

The pressure generator 4140 is under the control of the therapy devicecontroller 4240.

In other forms, a pressure generator 4140 may be a piston-driven pump, apressure regulator connected to a high pressure source (e.g. compressedair reservoir), or a bellows.

7.4.1.4 Transducer(s)

Transducers may be internal of the RPT device, or external of the RPTdevice. External transducers may be located for example on or form partof the air circuit, e.g., the patient interface. External transducersmay be in the form of non-contact sensors such as a Doppler radarmovement sensor that transmit or transfer data to the RPT device.

In one form of the present technology, one or more transducers 4270 arelocated upstream and/or downstream of the pressure generator 4140. Theone or more transducers 4270 may be constructed and arranged to measureproperties such as a flow rate, a pressure or a temperature at thatpoint in the pneumatic path.

In one form of the present technology, one or more transducers 4270 maybe located proximate to the patient interface 3000.

In one form, a signal from a transducer 4270 may be filtered, such as bylow-pass, high-pass or band-pass filtering.

7.4.1.4.1 Flow Rate Sensor

A flow rate sensor 4274 in accordance with the present technology may bebased on a differential pressure transducer, for example, an SDP600Series differential pressure transducer from SENSIRION.

In one form, a signal representing a flow rate such as a total flow rateQt from the flow rate sensor 4274 is received by the central controller4230.

7.4.1.4.2 Pressure Sensor

A pressure sensor 4272 in accordance with the present technology islocated in fluid communication with the pneumatic path. An example of asuitable pressure transducer is a sensor from the HONEYWELL ASDX series.An alternative suitable pressure transducer is a sensor from the NPASeries from GENERAL ELECTRIC.

In one form, a signal from the pressure sensor 4272 is received by thecentral controller 4230.

7.4.1.4.3 Motor Speed Transducer

In one form of the present technology a motor speed transducer 4276 isused to determine a rotational velocity of the motor 4144 and/or theblower 4142. A motor speed signal from the motor speed transducer 4276may be provided to the therapy device controller 4240. The motor speedtransducer 4276 may, for example, be a speed sensor, such as a Halleffect sensor.

7.4.1.5 Anti-Spill Back Valve

In one form of the present technology, an anti-spill back valve islocated between the humidifier 5000 and the pneumatic block 4020. Theanti-spill back valve is constructed and arranged to reduce the riskthat water will flow upstream from the humidifier 5000, for example tothe motor 4144.

7.4.1.6 Air Circuit

An air circuit 4170 in accordance with an aspect of the presenttechnology is a conduit or a tube constructed and arranged in use toallow a flow of air to travel between two components such as thepneumatic block 4020 and the patient interface 3000.

In particular, the air circuit 4170 may be in fluid connection with theoutlet of the pneumatic block and the patient interface. The air circuitmay be referred to as an air delivery tube. In some cases there may beseparate limbs of the circuit for inspiration and expiration. In othercases a single limb is used.

In some forms, the air circuit 4170 may comprise one or more heatingelements configured to heat air in the air circuit, for example tomaintain or raise the temperature of the air. The heating element may bein a form of a heated wire circuit, and may comprise one or moretransducers, such as temperature sensors. In one form, the heated wirecircuit may be helically wound around the axis of the air circuit 4170.The heating element may be in communication with a controller such as acentral controller 4230 or a humidifier controller 5250. One example ofan air circuit 4170 comprising a heated wire circuit is described inUnited States Patent Application No. US/2011/0023874, which isincorporated herewithin in its entirety by reference.

7.4.1.7 Oxygen Delivery

In one form of the present technology, supplemental oxygen 4180 isdelivered to one or more points in the pneumatic path, such as upstreamof the pneumatic block 4020, to the air circuit 4170 and/or to thepatient interface 3000.

7.4.2 RPT Device Electrical Components

7.4.2.1 Power Supply

A power supply 4210 may be located internal or external of the externalhousing 4010 of the RPT device 4000.

In one form of the present technology, power supply 4210 provideselectrical power to the RPT device 4000 only. In another form of thepresent technology, power supply 4210 provides electrical power to bothRPT device 4000 and humidifier 5000.

7.4.2.2 Input Devices

In one form of the present technology, an RPT device 4000 includes oneor more input devices 4220 in the form of buttons, switches or dials toallow a person to interact with the device. The buttons, switches ordials may be physical devices, or software devices accessible via atouch screen. The buttons, switches or dials may, in one form, bephysically connected to the external housing 4010, or may, in anotherform, be in wireless communication with a receiver that is in electricalconnection to the central controller 4230.

In one form, the input device 4220 may be constructed and arranged toallow a person to select a value and/or a menu option.

7.4.2.3 Central Controller

In one form of the present technology, the central controller 4230 isone or a plurality of processors suitable to control an RPT device 4000.

Suitable processors may include an x86 INTEL processor, a processorbased on ARM® Cortex®-M processor from ARM Holdings such as an STM32series microcontroller from ST MICROELECTRONIC. In certain alternativeforms of the present technology, a 32-bit RISC CPU, such as an STR9series microcontroller from ST MICROELECTRONICS or a 16-bit RISC CPUsuch as a processor from the MSP430 family of microcontrollers,manufactured by TEXAS INSTRUMENTS may also be suitable.

In one form of the present technology, the central controller 4230 is adedicated electronic circuit.

In one form, the central controller 4230 is an application-specificintegrated circuit. In another form, the central controller 4230comprises discrete electronic components.

The central controller 4230 may be configured to receive input signal(s)from one or more transducers 4270, and one or more input devices 4220.

The central controller 4230 may be configured to provide outputsignal(s) to one or more of an output device 4290, a therapy devicecontroller 4240, a data communication interface 4280 and humidifiercontroller 5250.

In some forms of the present technology, the central controller 4230 isconfigured to implement the one or more methodologies described herein,such as the one or more algorithms 4300 expressed as computer programsstored in a non-transitory computer readable storage medium, such asmemory 4260. In some forms of the present technology, the centralcontroller 4230 may be integrated with an RPT device 4000. However, insome forms of the present technology, some methodologies may beperformed by a remotely located device. For example, the remotelylocated device may determine control settings for a ventilator or detectrespiratory related events by analysis of stored data such as from anyof the sensors described herein.

7.4.2.4 Clock

The RPT device 4000 may include a clock 4232 that is connected to thecentral controller 4230.

7.4.2.5 Therapy Device Controller

In one form of the present technology, therapy device controller 4240 isa therapy control module 4330 that forms part of the algorithms 4300executed by the central controller 4230.

In one form of the present technology, therapy device controller 4240 isa dedicated motor control integrated circuit. For example, in one form aMC33035 brushless DC motor controller, manufactured by ONSEMI is used.

7.4.2.6 Protection Circuits

The one or more protection circuits 4250 in accordance with the presenttechnology may comprise an electrical protection circuit, a temperatureand/or pressure safety circuit.

7.4.2.7 Memory

In accordance with one form of the present technology the RPT device4000 includes memory 4260, e.g., non-volatile memory. In some forms,memory 4260 may include battery powered static RAM. In some forms,memory 4260 may include volatile RAM.

Memory 4260 may be located on the PCBA 4202. Memory 4260 may be in theform of EEPROM, or NAND flash.

Additionally or alternatively, RPT device 4000 includes a removable formof memory 4260, for example a memory card made in accordance with theSecure Digital (SD) standard.

In one form of the present technology, the memory 4260 acts as anon-transitory computer readable storage medium on which is storedcomputer program instructions expressing the one or more methodologiesdescribed herein, such as the one or more algorithms 4300. The memory4260 may also act as a volatile or non-volatile storage medium for dataacquired, collected, used or generated as one or more of themethodologies described herein are executed as instructions by one ormore processors.

7.4.2.8 Data Communication Systems

In one form of the present technology, a data communication interface4280 is provided, and is connected to the central controller 4230. Datacommunication interface 4280 may be connectable to a remote externalcommunication network 4282 and/or a local external communication network4284. The remote external communication network 4282 may be connectableto a remote external device 4286. The local external communicationnetwork 4284 may be connectable to a local external device 4288.

In one form, data communication interface 4280 is part of the centralcontroller 4230. In another form, data communication interface 4280 isseparate from the central controller 4230, and may comprise anintegrated circuit or a processor.

In one form, remote external communication network 4282 is the Internet.The data communication interface 4280 may use wired communication (e.g.via Ethernet, or optical fibre) or a wireless protocol (e.g. CDMA, GSM,LTE) to connect to the Internet.

In one form, local external communication network 4284 utilises one ormore communication standards, such as Bluetooth, or a consumer infraredprotocol.

In one form, remote external device 4286 is one or more computers, forexample a cluster of networked computers. In one form, remote externaldevice 4286 may be virtual computers, rather than physical computers. Ineither case, such a remote external device 4286 may be accessible to anappropriately authorised person such as a clinician.

The local external device 4288 may be a personal computer, mobile phone,tablet or remote control.

7.4.2.9 Output Devices Including Optional Display, Alarms

An output device 4290 in accordance with the present technology may takethe form of one or more of a visual, audio and haptic unit. A visualdisplay may be a Liquid Crystal Display (LCD) or Light Emitting Diode(LED) display.

7.4.2.9.1 Display Driver

A display driver 4292 receives as an input the characters, symbols, orimages intended for display on the display 4294, and converts them tocommands that cause the display 4294 to display those characters,symbols, or images.

7.4.2.9.2 Display

A display 4294 is configured to visually display characters, symbols, orimages in response to commands received from the display driver 4292.For example, the display 4294 may be an eight-segment display, in whichcase the display driver 4292 converts each character or symbol, such asthe figure “0”, to eight logical signals indicating whether the eightrespective segments are to be activated to display a particularcharacter or symbol.

7.4.3 RPT Device Algorithms

7.4.3.1 Pre-Processing Module

A pre-processing module 4310 in accordance with one form of the presenttechnology receives as an input a signal from a transducer 4270, forexample a flow rate sensor 4274 or pressure sensor 4272, and performsone or more process steps to calculate one or more output values thatwill be used as an input to another module, for example a therapy enginemodule 4320.

In one form of the present technology, the output values include theinterface or mask pressure Pm, the respiratory flow rate Qr, and theleak flow rate Ql.

In various forms of the present technology, the pre-processing module4310 comprises one or more of the following algorithms: pressurecompensation 4312, vent flow rate estimation 4314, leak flow rateestimation 4316, and respiratory flow rate estimation 4318.

7.4.3.1.1 Pressure Compensation

In one form of the present technology, a pressure compensation algorithm4312 receives as an input a signal indicative of the pressure in thepneumatic path proximal to an outlet of the pneumatic block. Thepressure compensation algorithm 4312 estimates the pressure drop throughthe air circuit 4170 and provides as an output an estimated pressure,Pm, in the patient interface 3000.

7.4.3.1.2 Vent Flow Rate Estimation

In one form of the present technology, a vent flow rate estimationalgorithm 4314 receives as an input an estimated pressure, Pm, in thepatient interface 3000 and estimates a vent flow rate of air, Qv, from avent 3400 in a patient interface 3000.

7.4.3.1.3 Leak Flow Rate Estimation

In one form of the present technology, a leak flow rate estimationalgorithm 4316 receives as an input a total flow rate, Qt, and a ventflow rate Qv, and provides as an output an estimate Ql of the leak flowrate. In one form, the leak flow rate estimation algorithm 4316estimates the leak flow rate Ql by calculating an average of thedifference between total flow rate Qt and vent flow rate Qv over aperiod sufficiently long to include several breathing cycles, e.g. about10 seconds.

In one form, the leak flow rate estimation algorithm 4316 receives as aninput a total flow rate Qt, a vent flow rate Qv, and an estimatedpressure, Pm, in the patient interface 3000, and provides as an output aleak flow rate Ql, by calculating a leak conductance, and determining aleak flow rate Ql to be a function of leak conductance and pressure, Pm.Leak conductance is calculated as the quotient of low pass filterednon-vent flow rate equal to the difference between total flow rate Qtand vent flow rate Qv, and low pass filtered square root of pressure Pm,where the low pass filter time constant has a value sufficiently long toinclude several breathing cycles, e.g. about 10 seconds. The leak flowrate Ql may be estimated as the product of leak conductance and afunction of pressure, Pm.

7.4.3.1.4 Respiratory Flow Rate Estimation

In one form of the present technology, a respiratory flow rateestimation algorithm 4318 receives as an input a total flow rate, Qt, avent flow rate, Qv, and a leak flow rate, Ql, and estimates arespiratory flow rate of air, Qr, to the patient, by subtracting thevent flow rate Qv and the estimated leak flow rate Ql from the totalflow rate Qt.

7.4.3.2 Therapy Engine Module

In one form of the present technology, a therapy engine module 4320receives as inputs one or more of a pressure, Pm, in a patient interface3000, and a respiratory flow rate of air to a patient, Qr, and providesas an output one or more therapy parameters.

In one form of the present technology, a therapy parameter is atreatment pressure Pt.

In various forms, the therapy engine module 4320 comprises one or moreof the following algorithms: phase determination 4321, waveformdetermination 4322, ventilation determination 4323, inspiratory flowlimitation detection 4324, apnea detection 4325, inspiratory M-shapedetection 4326, airway patency determination 4327, typical recentventilation determination 4328, and therapy parameter determination4329.

7.4.3.2.1 Phase Determination

In one form of the present technology, a phase determination algorithm4321 receives as an input a signal indicative of respiratory flow rate,Qr, and provides as an output a phase Φ of a current breathing cycle ofa patient 1000.

In some forms, known as discrete phase determination, the phase output Φis a discrete variable. One implementation of discrete phasedetermination provides a bi-valued phase output Φ with values of eitherinspiration or expiration, for example represented as values of 0 and0.5 revolutions respectively, upon detecting the start of spontaneousinspiration and expiration respectively. RPT devices 4000 that “trigger”and “cycle” effectively perform discrete phase determination, since thetrigger and cycle instants are the instants at which the phase changesfrom expiration to inspiration and from inspiration to expiration,respectively. In one implementation of bi-valued phase determination,the phase output Φ is determined to have a discrete value of 0(indicative of inspiration) when the respiratory flow rate Qr exceeds a“trigger threshold” (thereby triggering the RPT device 4000 to deliver a“spontaneous breath”), and a discrete value of 0.5 revolutions(indicative of expiration) when the respiratory flow rate Qr falls belowa “cycle threshold” (thereby “spontaneously cycling” the RPT device4000). In some such implementations, the trigger and cycle thresholdsmay vary with time during a breath according to respective trigger andcycle threshold functions. Such functions are described in the PatentCooperation Treaty patent application number PCT/AU2005/000895,published as WO 2006/000017, to ResMed Limited, the entire contents ofwhich are herein incorporated by reference.

In some such implementations, cycling may be prevented during a“refractory period” (denoted as Timin) after the last trigger instant,and, absent spontaneous cycling, must occur within an interval (denotedas Timax) after the last trigger instant. The values of Timin and Timaxare settings of the RPT device 4000, and may be set for example byhard-coding during configuration of the RPT device 4000 or by manualentry through the input device 4220.

In other forms, known as continuous phase determination, the phaseoutput Φ is a continuous variable, for example varying from 0 to 1revolutions, or 0 to 2π radians. RPT devices 4000 that performcontinuous phase determination may trigger and cycle when the continuousphase reaches 0 and 0.5 revolutions, respectively. In one implementationof continuous phase determination, the inspiratory time Ti and theexpiratory time Te are first estimated from the respiratory flow rateQr. The phase Φ is then determined as the half the proportion of theinspiratory time Ti that has elapsed since the previous trigger instant,or 0.5 revolutions plus half the proportion of the expiratory time Tethat has elapsed since the previous cycle instant (whichever was morerecent).

In some implementations, suitable for ventilation therapy (describedbelow), the phase determination algorithm 4321 is configured to triggereven when the respiratory flow rate Qr is insignificant, such as duringan apnea. As a result, the RPT device 4000 delivers “backup breaths” inthe absence of spontaneous respiratory effort from the patient 1000. Forsuch forms, known as spontaneous/timed (ST) modes, the phasedetermination algorithm 4321 may make use of a “backup rate” Rb. Thebackup rate Rb is a setting of the RPT device 4000, and may be set forexample by hard-coding during configuration of the RPT device 4000 or bymanual entry through the input device 4220.

A phase determination algorithm 4321 (either discrete, or continuous)may implement ST modes using the backup rate Rb in a manner known astimed backup. Timed backup may be implemented as follows: the phasedetermination algorithm 4321 attempts to detect the start of inspirationdue to spontaneous respiratory effort, for example by comparing therespiratory flow rate Qr with a trigger threshold as described above. Ifthe start of spontaneous inspiration is not detected within an intervalafter the last trigger instant whose duration is equal to the reciprocalor inverse of the backup rate Rb (an interval referred to as the backuptiming threshold, Tbackup), the phase determination algorithm 4321 setsthe phase output Φ to value of 0, thereby triggering the RPT device 4000to deliver a backup breath. The phase determination algorithm 4321 thenattempts to detect the start of spontaneous expiration, for example bycomparing the respiratory flow rate Qr with a cycle threshold asdescribed above. The cycle threshold for backup breaths may be differentfrom the cycle threshold for spontaneous breaths. As with spontaneousbreaths, spontaneous cycling during backup breaths may be preventedduring a “refractory period” of duration Timin after the last triggerinstant.

As with spontaneous breaths, if during a backup breath the start ofspontaneous expiration is not detected within Timax seconds after thelast trigger instant, the phase determination algorithm 4321 sets thephase output Φ to value of 0.5, thereby cycling the RPT device 4000. Thephase determination algorithm 4321 then attempts to detect the start ofspontaneous inspiration by comparing the respiratory flow rate Qr with atrigger threshold as described above.

7.4.3.2.2 Waveform Determination

In one form of the present technology, the waveform determinationalgorithm 4322 provides an approximately constant treatment pressurethroughout a respiratory cycle of a patient.

In other forms of the present technology, the waveform determinationalgorithm 4322 controls the pressure generator 4140 to provide atreatment pressure Pt that varies throughout a respiratory cycle of apatient according to a waveform template.

In one form of the present technology, a waveform determinationalgorithm 4322 provides a waveform template Π(Φ) with values in therange [0, 1] on the domain of phase values Φ provided by the phasedetermination algorithm 4321 to be used by the waveform determinationalgorithm 4322.

In one form, suitable for either discrete or continuously-valued phase,the waveform template Π(Φ) is a square-wave template, having a value of1 for values of phase up to and including 0.5 revolutions, and a valueof 0 for values of phase above 0.5 revolutions. In one form, suitablefor continuously-valued phase, the waveform template Π(Φ) comprises twosmoothly curved portions, namely a smoothly curved (e.g. raised cosine)rise from 0 to 1 for values of phase up to 0.5 revolutions, and asmoothly curved (e.g. exponential) decay from 1 to 0 for values of phaseabove 0.5 revolutions.

In some forms of the present technology, the waveform determinationalgorithm 4322 selects a waveform template Π(Φ) from a library ofwaveform templates, dependent on a setting of the RPT device 4000. Eachwaveform template Π(Φ) in the library may be provided as a lookup tableof values Π against phase values Φ. In other forms, the waveformdetermination algorithm 4322 computes a waveform template Π(Φ) “on thefly” using a predetermined functional form, possibly parametrised by oneor more parameters (e,g. a rise time and a fall time). The parameters ofthe functional form may be predetermined or dependent on a current stateof the patient 1000.

In some forms of the present technology, suitable for discrete bi-valuedphase of either inspiration (Φ=0 revolutions) or expiration (Φ=0.5revolutions), the waveform determination algorithm 4322 computes awaveform template Π “on the fly” as a function of both discrete phase Φand time t measured since the most recent trigger instant. In one suchform, the waveform determination algorithm 4322 computes the waveformtemplate Π(Φ, t) in two portions (inspiratory and expiratory) asfollows:

${\Pi\left( {\Phi,t} \right)} = \left\{ \begin{matrix}{{\Pi_{i}(t)},} & {\Phi = 0} \\{{\Pi_{e}\left( {t - {Ti}} \right)},} & {\Phi = 0.5}\end{matrix} \right.$

where Π_(i)(t) and Π_(e)(t) are inspiratory and expiratory portions ofthe waveform template Π(Φ, t).

In one such form, the inspiratory portion Π_(i)(t) of the waveformtemplate is a smooth rise from 0 to 1 in two continuous sections:

-   -   a linear rise up to ⅔ for the first half of a parameter known as        the “time scale”;    -   a parabolic rise up to 1 for the second half of the time scale.

The “rise time” of such an inspiratory portion Π_(i)(t) may be definedas the time taken for Π_(i)(t) to rise to a value of 0.875.

The expiratory portion Π_(e)(t) of the waveform template is a smoothfall from 1 to 0 in two continuous parabolic sections, with aninflection point between 25% and 50% of the time scale. The “fall time”of such an expiratory portion Π_(e)(t) may be defined as the time takenfor Π_(e)(t) to fall to a value of 0.125.

7.4.3.2.3 Ventilation Determination

In one form of the present technology, a ventilation determinationalgorithm 4323 receives an input a respiratory flow rate Qr, anddetermines a measure Vent indicative of current patient ventilation.

In some implementations, the ventilation determination algorithm 4323computes Vent as an “instantaneous ventilation” Vint, which is half theabsolute value of the respiratory flow rate signal Qr.

In some implementations, the ventilation determination algorithm 4323computes Vent as a “very fast ventilation” VveryFast by filtering theinstantaneous ventilation Vint by a low-pass filter such as a fourthorder Bessel low-pass filter with a corner frequency of approximately0.10 Hz. This is equivalent to a time constant of approximately tenseconds.

In some implementations, the ventilation determination algorithm 4323computes a Vent as a “fast ventilation” Vfast by filtering theinstantaneous ventilation Vint by a low-pass filter such as a fourthorder Bessel low-pass filter with a corner frequency of approximately0.05 Hz. This is equivalent to a time constant of approximately twentyseconds.

In some implementations of the present technology, the ventilationdetermination algorithm 4323 determines Vent as a measure of alveolarventilation. Alveolar ventilation is a measure of how much air isactually reaching the gas exchange surfaces of the respiratory system ina given time. Because the respiratory system of the patient includes asignificant “anatomical dead space”, i.e. volume in which gas exchangedoes not take place, the alveolar ventilation is less than the “gross”ventilation values that the above calculations that operate directly onthe respiratory flow rate Qr will produce, but is a more accuratemeasure of the respiratory performance of a patient.

In such implementations, the ventilation determination algorithm 4323may determine the instantaneous alveolar ventilation to be either zeroor half the absolute value of the respiratory flow rate Qr. Theconditions under which the instantaneous alveolar ventilation is zeroare:

-   -   When the respiratory flow rate changes from non-negative to        negative, or    -   When the respiratory flow rate changes from negative to        non-negative, and    -   After the respiratory flow rate has changed sign, for the period        during which the absolute value of the integral of the        respiratory flow rate Qr is less than the patient's anatomical        dead space volume.

The patient's anatomical dead space volume may be a setting of the RPTdevice 4000, set for example by hard-coding during configuration of theRPT device 4000 or by manual entry through the input device 4220.

In some such implementations, the ventilation determination algorithm4323 may compute Vent as a “very fast alveolar ventilation” and/or a“fast alveolar ventilation” by low-pass filtering the instantaneousalveolar ventilation using the respective low-pass filters describedabove.

In what follows the word “alveolar” is omitted but it may be assumed tobe present in some implementations of the therapy engine module 4320.That is, mentions of “ventilation” and “tidal volume” in the subsequentdescription may be taken to apply to alveolar ventilation and alveolartidal volume as well as “gross” ventilation and tidal volume.

7.4.3.2.4 Inspiratory Flow Limitation Determination

In one form of the present technology, the therapy engine module 4320executes one or more algorithms to determine the extent of flowlimitation, sometimes referred to as partial upper airway obstruction,in the inspiratory portion of the respiratory flow rate waveform (hereinsometimes shortened to the “inspiratory waveform”). In one form, theflow limitation determination algorithm 4324 receives as an input arespiratory flow rate signal Qr and provides as an output a measure ofthe extent to which each inspiratory waveform exhibits flow limitation.

A normal inspiratory waveform is rounded, close to sinusoidal in shape(see FIG. 6A). With sufficient upper airway muscle tone (or EPAP), theairway acts essentially as a rigid tube, where flow increases inresponse to increased breathing effort (or external ventilatoryassistance). In some situations (e.g. sleep, sedation) the upper airwaymay be collapsible, such as in response to sub-atmospheric pressurewithin it from breathing effort, or even from applied ventilation. Thiscan lead to either full obstruction (apneas), or a phenomenon known as‘flow limitation’. The term ‘flow limitation’ includes behaviour whereincreased breathing effort simply induces increased narrowing of theairway, such that inspiratory flow becomes limited at a constant value,independent of effort (“Starling resistor behaviour”). Therefore theinspiratory flow rate curve exhibits a flattened shape (see FIG. 6B).

In reality, upper airway behaviour is even more complicated, and thereis a wide variety of flow shapes that are indicative of upperairway-related inspiratory flow limitation, and an even wider variety inthe presence of external ventilatory assistance (see FIGS. 6C to 6F).For this reason, the flow limitation determination algorithm 4324 mayrespond to one or more of the following kinds of inspiratory flowlimitation: “classical flatness” (see FIG. 6B), “chairness” (see FIG.6C), and “reverse chairness” (see FIG. 6D). (“M-shape” (see FIGS. 6E and6F) is dealt with separately, using M-shape detection algorithm 4326.)

FIG. 7A is a flow chart illustrating a method 7000 that may be used tocompute a measure of flow limitation of the inspiratory portion of therespiratory flow rate waveform as part of the inspiratory flowlimitation determination algorithm 4324 in one form of the presenttechnology.

The method 7000 starts at step 7010, which computes a number of “centralpart features” CF1 to CF8 based on the central part of the inspiratorywaveform, including the central slope, the central deviation, thecentral concavity, and the extents to which the waveform has a largeinitial section and a large final section (before and after the centralpart, approximately). Step 7010 is described in detail below withreference to FIG. 7B.

The next step 7020 combines the central part features CF1 to CF8computed at step 7010 using fuzzy logic to compute a number of flowlimitation fuzzy truth variables FL1 to FL6 indicating degrees ofsimilarity between the inspiratory waveform and respective stereotypicalflow-limited (partially obstructed) inspiratory waveforms. Thesestereotypes may include: moderately flat, possibly concave; mildlynegative slope, indicating high inspiratory resistance; very flat; flator M-shaped with large initial peak; flattish, possibly with a somewhatnegative slope, with a peak in the post-central part; and concave andflattish. Step 7020 is described in detail below with reference to FIG.7C.

The method 7000 then at step 7030 computes a fuzzy truth variablefuzzyLateFlatness that indicates a degree of “chairness” of theinspiratory waveform, i.e. a combination of early peak, late linearity,and moderate late slope (as illustrated in FIG. 6C). Step 7030 isdescribed in detail below with reference to FIG. 7E.

Finally at step 7040, the computed flow limitation fuzzy truth variablesFL1 to FL6 (from step 7020) and the fuzzy truth variablefuzzyLateFlatness (from step 7030) are all combined using a “fuzzy OR”operation to compute a fuzzy truth variable flowLimitation indicatingthe extent of inspiratory flow limitation. The fuzzy “OR” takes themaximum of the fuzzy truth variables, so the fuzzy truth variableflowLimitation indicates the degree of similarity to the one of thestereotypical flow-limited inspiratory waveforms to which theinspiratory waveform is most similar.

By basing the detection of flow limitation primarily on featuresextracted from the central part of the inspiratory waveform, the method7000 is more robust to mismodelled leak than other methods of detectinginspiratory flow limitation. This is because mismodelled leak tends toinduce a (usually, though not exclusively) positive offset to therespiratory flow rate signal Qr that is a function of the mask pressurePm, which tends to be generally constant during the central and laterparts of the inspiratory waveform (e.g. once the rise time has passed).The offset to the respiratory flow rate signal Qr induced by mismodelledleak therefore also tends to be constant during the central and laterparts of the inspiratory waveform.

FIG. 7B is a flow chart illustrating a method 7100 that may be used toimplement step 7010 of the method 7000 in one form of the presenttechnology. The method 7100 receives as parameters a “low fraction” anda “high fraction” of the duration of the inspiratory waveform. The“central part” of the inspiratory waveform used by the method 7100extends from the low fraction multiplied by the duration of theinspiratory waveform to the high fraction multiplied by the duration ofthe inspiratory waveform. In one implementation of step 7010, the “lowfraction” passed to the method 7100 is 0.3, and the “high fraction” is0.85.

The method 7100 starts at step 7110, which constructs a notional “ramp”over the part of the inspiratory waveform before the central part, i.e.the “pre-central” part. The ramp extends from the start point of theinspiratory waveform to the start point of the central part.

Step 7120 then computes the amount of respiratory flow above the ramp bysumming the difference between the inspiratory waveform and the rampover the pre-central part. This amount is referred to asflowAboveRampToStartOfCentralPart.

The next step 7130 computes the first central part feature (CF1),propnAboveRampToStartOfCentralPart, by dividingflowAboveRampToStartOfCentralPart by the sum of all the inspiratorywaveform values. A markedly positive value ofpropnAboveRampToStartOfCentralPart is generally associated with an earlypeak of the inspiratory flow rate waveform. A negative value indicates avery gentle rise of flow to the central part, and is unlikely to beassociated with flow limitation or high inspiratory resistance.

Step 7140 follows, at which the method 7100 computes the second centralpart feature (CF2), centralSlope, over the central part. CentralSlope,which represents the slope of a linear approximation to the normalisedcentral part of the inspiratory waveform, may be computed by linearregression of the normalised central part. The normalising factor is themaximum of the mean of the complete inspiratory waveform, and theminimum normalising flow, which is defined in one implementation to be24 litres/minute.

Step 7150 then computes centralFlowsMean, the mean of the normalisedcentral part of the inspiratory waveform. Step 7150 then multipliescentralFlowsMean by the duration of the central part to obtain the thirdcentral part feature (CF3), centralPartDurationByFlowsMeanProduct.

The method 7100 proceeds to step 7160, which computes the fourth centralpart feature (CF4), postCentralActualAbovePredicted, a measure of howmuch the flow rate in the part of the inspiratory waveform after thecentral part (the post-central part) is above the linear approximationto the central part. Step 7160 computes postCentralActualAbovePredictedby summing the difference between the normalised inspiratory waveformvalues and the linear approximation extended over the post-central part,and dividing the sum by the duration of the post-central part and thevalue of centralFlowsMean. (In one implementation, if centralFlowsMeanis not positive, postCentralActualAbovePredicted is set to zero.) Anegative value of postCentralActualAbovePredicted generally indicatesnormal breathing, because in partial upper airway obstruction the flowrate is generally greater than or equal to the projection of the centralpart forward in time, whereas in normal breathing the flow rate afterthe central part almost always decreases more rapidly than in thecentral part. Put another way, in normal breathing the flow rate in thecentral part and the post-central part taken together is generallyconvex, whereas in flow-limited breathing, the flow rate in the centralpart taken together with most of the post-central part is generally notconvex.

The next step 7170 computes the fifth central part feature (CF5),centralDeviation, a measure of the variation of the actual inspiratorywaveform values from the linear approximation thereto over the centralpart. In one implementation of step 7170, centralDeviation is computedas the square root of the sum of the squares of the difference betweenthe normalised flow rate and the linear approximation thereto (computedfrom centralFlowsMean and centralSlope), over the central part. Amarkedly positive value of centralDeviation indicates significantnon-linearity in the central part.

Finally, at step 7180 the method 7100 computes the sixth central partfeature (CF6), centralConcavity, a measure of the concavity of thecentral part. A positive value of centralConcavity indicates the centralpart is roughly concave upwards. In one implementation of step 7180,centralConcavity is computed as a measure of similarity between thenormalised, mean-subtracted inspiratory waveform and a V-shaped functionover the central part. In one implementation of step 7180, the measureof similarity is computed as the sum over the central part of theproducts of the normalised, mean-subtracted inspiratory waveform and theV-shaped function, divided by the autocorrelation of the V-shapedfunction. Other measures of concavity may also be used at step 7180 tocompute centralConcavity, e.g. the curvature of a parabolicapproximation to the central part. Another measure of concavity is theinner product of the central part with the second derivative of aGaussian function with a suitable scaling factor, over a suitable range,e.g. that corresponding to +/−2 standard deviations. This mayalternatively be implemented by taking the second derivative of the flowat the centre of the low-pass filtered central part, where the low-passfiltering has been performed by convolution with a Gaussian function,possibly with appropriate windowing. Other low-pass filters may be used,followed by a standard numerical method for calculating the secondderivative on the filtered waveform.

To compute the seventh and eighth central part features, step 7010 mayrepeat the method 7100 with a different definition of the central part,i.e. different values of the parameters “low fraction” and “highfraction”. In one such implementation, the “low fraction” for the seconditeration of the method 7100 is 0.2, and the “high fraction” is 0.8,both of which are lower than the corresponding fractions defining thecentral part over which the central part features CF1 to CF6 werecomputed, and which therefore define an earlier portion of theinspiratory waveform. The seventh central part feature (CF7), namedConcaveAndFlattishCentralSlope, is the value of centralSlope returned bystep 7140 of the second iteration of the method 7100. The value ofConcaveAndFlattishCentralSlope is subtly different from the value ofcentralSlope (CF2) from being computed over an earlier portion of theinspiratory waveform.

The eighth central part feature (CF8), namedConcaveAndFlattishCentralConcavity, is the value of centralConcavityreturned by step 7180 of the second iteration of the method 7100. Thevalue of ConcaveAndFlattishCentralConcavity is subtly different from thevalue of centralConcavity (CF6) from being computed over an earlierportion of the inspiratory waveform.

By basing the computation of central part features CF1 to CF8 generallyon normalised quantities, the method 7100 makes itself more robust tooffsets in the inspiratory waveform induced by mismodelled leak.

FIG. 7C is a flow chart illustrating a method 7200 that may be used toimplement step 7020 of the method 7000 in one form of the presenttechnology. As mentioned above, step 7020 combines the central partfeatures CF1 to CF8 computed at step 7010, such as by using fuzzy logic,to compute a number of flow limitation fuzzy truth variables FL1 to FL6indicating the similarity of the inspiratory waveform to respectivestereotypical flow-limited inspiratory waveforms.

Each step 7210 to 7260 operates independently on some subset of theeight central part features CF1 to CF8 to compute one of six flowlimitation fuzzy truth variables FL1 to FL6. For this reason, steps 7210to 7260 may be carried out in parallel or any convenient order.

Step 7210 computes the flow limitation fuzzy truth variable FL1, namedflattishMaybeConcave, as the fuzzy AND of three fuzzy truth variablescentralSlopeFlattishMaybeConcave, centralDeviationFlattishMaybeConcave,and centralConcavityFlattishMaybeConcave, obtained from the central partfeatures centralSlope (CF2), centralDeviation (CF5) and centralConcavity(CF6) respectively. The three fuzzy truth variables that contribute toflattishMaybeConcave indicate respectively that the central part of theinspiratory waveform is moderately flat (approximately horizontal andreasonably linear) and slightly concave upwards. In one implementation,step 7210 computes centralSlopeFlattishMaybeConcave,centralDeviationFlattishMaybeConcave, andcentralConcavityFlattishMaybeConcave as follows:centralSlopeFlattishMaybeConcave=FuzzyMember(centralSlope,−0.2,False,−0.1,True,0.1,True,0.2,False)centralDeviationFlattishMaybeConcave=FuzzyMember(centralDeviation,0.05,True,0.1,False)centralConcavityFlattishMaybeConcave=FuzzyMember(centralConcavity,−0.2,False,0.0,True)

Step 7220 computes the flow limitation fuzzy truth variable FL2, namedmildNegSlopeHighInspResistance, from the six central part features CF1to CF6. The flow limitation fuzzy truth variablemildNegSlopeHighInspResistance indicates that the inspiratory waveformhas mildly negative slope in the central part indicating highinspiratory resistance.

FIG. 7D is a flow chart illustrating a method 7300 that may be used toimplement step 7220 of the method 7200 in one form of the presenttechnology.

The method 7300 starts at step 7310, which checks whether the centralpart feature centralPartDurationByFlowsMeanProduct (CF3) is greater than0, or the central part feature centralSlope (CF2) is between [−0.6,−0.1]. If not (“N”), the method 7300 proceeds to step 7320, which setsmildNegSlopeHighInspResistance to False, because either the centralslope is not mildly negative or the central part is too small to beuseful.

Otherwise (“Y”), the method 7300 commences computing four fuzzy truthvariables which will be combined using a fuzzy AND operation to yieldmildNegSlopeHighInspResistance.

This computation starts at step 7330, which computes an intermediatefuzzy truth variable pairIsHigh from the central part featurepropnAboveRampToStartOfCentralPart (CF1) such that pairIsHigh goes Trueas the pre-central part becomes “peakier”. In one implementation, step7330 computes pairIsHigh as follows:

pairIsHigh = FuzzyMember(    propnAboveRampToStartOfCentralPart,       0.08, False, 0.12, True)

Step 7340 then computes a slope-dependent deviation threshold frompairIsHigh and centralSlope. In one implementation, step 7340 computesslopeDependentDeviationThreshold as follows:slopeDependentDeviationThreshold=pairIsHigh*0.08+(1−pairIsHigh)*(0.04+(centralSlope+0.5)*0.1)

At the next step 7350, the method 7300 computes the first of the fourfuzzy truth variables that contribute to mildNegSlopeHighInspResistance,namely centralDeviationLow, using the central part featurecentralDeviation (CF5) minus the slope-dependent deviation thresholdjust computed. The value of centralDeviationLow becomes False as thisdifference increases. In one implementation, step 7350 computescentralDeviationLow as follows:

centralDeviationLow = FuzzyMember(   centralDeviation −slopeDependentDeviationThreshold,         0.0, True, 0.01, False        )

Step 7360 then computes the second of the four fuzzy truth variablesthat contribute to mildNegSlopeHighInspResistance, namelycentralConcavityModPositive, from the central part featurecentralConcavity (CF6), such that centralConcavityModPositive is truewhen the concavity of the central part is moderately positive. In oneimplementation, step 7360 computes centralConcavityModPositive asfollows:centralConcavityModPositive=FuzzyMember(centralConcavity,−0.05,False,0.0,True,0.3,True,0.5,False)

The next step 7370 computes the third of the four fuzzy truth variablesthat contribute to mildNegSlopeHighInspResistance, namelypostCentralActualAbovePredMildNegSlope, from the central part featurepostCentralActualAbovePredicted (CF4), such thatpostCentralActualAbovePredMildNegSlope becomes true aspostCentralActualAbovePredicted increases. In one implementation, step7370 computes postCentralActualAbovePredMildNegSlope as follows:postCentralActualAbovePredMildNegSlope=FuzzyMember(postCentralActualAbovePredicted,−0.4,False,−0.3,True)

Step 7375 follows, at which the method 7300 computes an exponentialdecay constant exponentialDecayConstant from the central part featurescentralSlope (CF2) and centralPartDurationByFlowsMeanProduct (CF3). Inone implementation, step 7375 computes exponentialDecayConstant asfollows:exponentialDecayConstant=−centralSlope/centralPartDurationByFlowsMeanProduct

The next step 7380 computes the fourth of the four fuzzy truth variablesthat contribute to mildNegSlopeHighInspResistance, namelyexpDecayConstMildNegSlope, from exponentialDecayConstant. In oneimplementation, step 7380 computes expDecayConstMildNegSlope as follows:expDecayConstMildNegSlope=FuzzyMember(exponentialDecayConstant,−0.4,False,−0.2,True,0.2,True,0.4,False)

The final step 7390 of the method 7300 computesmildNegSlopeHighInspResistance as the fuzzy AND of the four fuzzy truthvariables centralDeviationLow, mildNegSlopeHighInspResistance,postCentralActualAbovePredMildNegSlope, and expDecayConstMildNegSlope.

Step 7230 computes the flow limitation fuzzy truth variable FL3, namedveryFlat, as the fuzzy AND of three fuzzy truth variablescentralSlopeVeryFlat, centralConcavityVeryFlat, andpropnAboveRampToStartOfCentralPartVeryFlat obtained from the centralpart features centralSlope (CF2), centralConcavity (CF6), andpropnAboveRampToStartOfCentralPart (CF1) respectively. The three fuzzytruth variables that contribute to veryFlat indicate respectively thatthe inspiratory waveform is approximately horizontal and not convexupwards in the central part, and not concave upwards in the pre-centralpart. In one implementation, step 7230 computes centralSlopeVeryFlat,centralConcavityVeryFlat, and propnAboveRampToStartOfCentralPartVeryFlatas follows:centralSlopeVeryFlat=FuzzyMember(centralSlope,−0.1,False,−0.05,True,0.05,True,0.1,False)centralConcavityVeryFlat=FuzzyMember(centralConcavity,−0.1,False,0.0,True)propnAboveRampToStartOfCentralPartVeryFlat=FuzzyMember(propnAboveRampToStartOfCentralPart,−0.05,False,0.0,True)

Step 7240 computes the flow limitation fuzzy truth variable FL4, namedflatAndLargeInitialPeakMShapes, as the fuzzy AND of three fuzzy truthvariables centralSlopeFlatAndLargeInitialPeakMShapes,centralConcavityFlatAndLargeInitialPeakMShapes, andpropnAboveRampToStartOfCentralPartFlatAndLargeInitialPeakMShapes,obtained from the central part features centralSlope (CF2),centralConcavity (CF6), and propnAboveRampToStartOfCentralPart (CF1)respectively. The three fuzzy truth variables that contribute toflatAndLargeInitialPeakMShapes indicate respectively that theinspiratory waveform is approximately horizontal or M-shaped in thecentral part, and has a large initial peak. In one implementation, step7240 computes centralSlopeFlatAndLargeInitialPeakMShapes,centralConcavityFlatAndLargeInitialPeakMShapes, andpropnAboveRampToStartOfCentralPartFlatAndLargeInitialPeakMShapes asfollows:centralSlopeFlatAndLargeInitialPeakMShapes=FuzzyMember(centralSlope,−0.3,False,−0.2,True,0.2,True,0.3,False)centralConcavityFlatAndLargeInitialPeakMShapes=FuzzyMember(centralConcavity,0.1,False,0.3,True)propnAboveRampToStartOfCentralPartFlatAndLargeInitialPeakMShapes=FuzzyMember(propnAboveRampToStartOfCentralPart,0.15,False,0.3,True)

Step 7250 computes the flow limitation fuzzy truth variable FL5, namedflatMNegSlopePostCentralHigh, using the following fuzzy logic operationson three intermediate fuzzy truth variables:flatMNegSlopePostCentralHigh=centralDeviationFlatMNegSlopePostCentralHighAND (flatNegSlope OR flatMoreNegSlopeHighPostCentral)

The fuzzy truth variable centralDeviationFlatMNegSlopePostCentralHigh isobtained from the central part feature centralDeviation (CF5) andindicates that the central part of the inspiratory waveform isreasonably linear. In one implementation, step 7250 computescentralDeviationFlatMNegSlopePostCentralHigh as follows:centralDeviationFlatMNegSlopePostCentralHigh=FuzzyMember(centralDeviation,0.1,True,0.2,False)

Step 7250 computes the fuzzy truth variable flatNegSlope as the fuzzyAND of two fuzzy truth variables centralSlopeFlatNegSlope andpostCentralActualAbovePredFlatNegSlope, obtained from the central partfeatures centralSlope (CF2) and postCentralActualAbovePredicted (CF4)respectively. The two fuzzy truth variables that contribute toflatNegSlope indicate respectively that the inspiratory waveform has aslight negative slope in the central part and is concave up in thepost-central part. In one implementation, step 7250 computescentralSlopeFlatNegSlope and postCentralActualAbovePredFlatNegSlope asfollows:centralSlopeFlatNegSlope=FuzzyMember(centralSlope,−0.45,False,−0.3,True,0.1,True,0.2,False)postCentralActualAbovePredFlatNegSlope=FuzzyMember(postCentralActualAbovePredicted,0.0,False,0.2,True)

Step 7250 computes the fuzzy truth variableflatMoreNegSlopeHighPostCentral as the fuzzy AND of two fuzzy truthvariables centralSlopeFlatMoreNegSlopeHighPostCentral andpostCentralActualAbovePredFlatMoreNegSlopeHighPostCentral, obtained fromthe central part features centralSlope (CF2) andpostCentralActualAbovePredicted (CF4) respectively. The two fuzzy truthvariables that contribute to flatMoreNegSlopeHighPostCentral indicaterespectively that the inspiratory waveform has a slightly more negativeslope in the central part than flatNegSlope and has a peak in thepost-central part. In one implementation, step 7250 computescentralSlopeFlatMoreNegSlopeHighPostCentral andpostCentralActualAbovePredFlatMoreNegSlopeHighPostCentral as follows:centralSlopeFlatMoreNegSlopeHighPostCentral=FuzzyMember(centralSlope,−0.55,False,−0.45,True,0.1,True,0.2,False)postCentralActualAbovePredFlatMoreNegSlopeHighPostCentral=FuzzyMember(postCentralActualAbovePredicted,0.2,False,0.3,True)

The fuzzy truth variable flatMNegSlopePostCentralHigh (FL5) computed bystep 7250 indicates that the inspiratory waveform is reasonably linearin the central part and either has a slight negative slope in thecentral part and is concave up in the post-central part, or has a morenegative slope in the central part and has a peak in the post-centralpart.

Step 7260 computes the flow limitation fuzzy truth variable FL6, namedconcaveAndFlattish, as the fuzzy AND of two fuzzy truth variablescentralSlopeConcaveAndFlattishFuzzy andcentralConcavityConcaveAndFlattishFuzzy, obtained from the central partfeatures centralSlopeConcaveAndFlattish (CF7) andcentralConcavityConcaveAndFlattish (CF8) respectively. The two fuzzytruth variables that contribute to concaveAndFlattish indicaterespectively that the central part of the inspiratory waveform isapproximately horizontal and concave upwards. The flow limitation fuzzytruth variable concaveAndFlattish (FL6) is somewhat different from thefirst flow limitation fuzzy truth variable flattishMaybeConcave (FL1) onaccount of its dependence on central part features CF7 and CF8 that arecomputed over a slightly earlier definition of the central part than CF2and CF6, which contribute to flattishMaybeConcave. That is,concaveAndFlattish indicates a slightly earlier appearance ofhorizontality and concavity than flattishMaybeConcave.

In one implementation, step 7260 computescentralSlopeConcaveAndFlattishFuzzy andcentralConcavityConcaveAndFlattishFuzzy as follows:centralSlopeConcaveAndFlattishFuzzy=FuzzyMember(centralSlopeConcaveAndFlattish,−0.4,False,−0.2,True,0.2,True,0.4,False)centralConcavityConcaveAndFlattishFuzzy=FuzzyMember(centralConcavityConcaveAndFlattish,0.3,False,0.4,True)

FIG. 7E is a flow chart illustrating a method 7400 that may be used toimplement step 7030 of the method 7000 in one form of the presenttechnology. As mentioned above, step 7030 computes a fuzzy truthvariable fuzzyLateFlatness that indicates that indicates a degree of“chairness” of the inspiratory waveform.

The method 7400 starts at step 7410, which checks whether theinspiratory time Ti is greater than an inspiratory time threshold and atypical recent ventilation Vtyp (returned by the typical recentventilation determination algorithm 4328, described below) is greaterthan zero. In one implementation, the inspiratory time threshold is 0.5.If not (“N”), step 7440 sets fuzzyLateFlatness to “fuzzy false”, and themethod 7400 concludes. Otherwise (“Y”), step 7420 computes start and endlocations of a “late section” of the inspiratory waveform. The aim ofstep 7420 is to define the late section so as to exclude any early peakin the inspiratory waveform. In one implementation, the late section isdefined using rise time and fall time parameters of the pressurewaveform template determined by the waveform determination algorithm4322. In one such implementation, the start location of the late sectionis 1.25 multiplied by the rise time, and the end location is one quarterof the fall time back from the end of the inspiratory waveform. Otherimplementations of step 7420 defining the “late section” may becontemplated. For example, step 7420 may test a section somewhat laterthan the above-defined “late section” for being a reasonableapproximation to a straight line with close to zero gradient, and ifthis is found, search backward to find the junction of thisapproximation with the rapid descent from the initial peak, using forexample a criterion which is approximately equivalent to a largepositive smoothed second derivative at the junction to locate thisjunction, then consider the late section to start approximately at thisjunction. This approach is more computationally intensive than theapproach previously described.

The method 7400 then checks whether the duration of the late sectiondefined at step 7420 is above a minimum fraction of the inspiratory timeTi. In one implementation the minimum fraction is 0.25. If not (“N”),the late section is deemed too short for a reliable analysis, and themethod 7400 proceeds to step 7440, which sets fuzzyLateFlatness to“fuzzy false”, and the method 7400 concludes. Otherwise (“Y”), the nextstep 7450 computes a normalising factor normFactorTypVent from thetypical recent ventilation Vtyp, such that normFactorTyptVent generallyincreases as Vtyp increases. In one implementation, suitable for whenVtyp is an alveolar ventilation, step 7450 computes normFactorTypVent asVtyp/4.5.

Step 7455 then computes a linear approximation to the inspiratorywaveform over the late section. Step 7455 may use linear regression tocompute the linear approximation, which is characterised by the meanflow flowMean, the slope flowSlope, and the root mean squared predictionerror rootMeanSqFlowPredErr. Step 7455 then computes normalised versionsof these three parameters by dividing each by the normalising factornormFactorTypVent:lateFlowMean=flowMean/normFactorTypVentlateSlope=flowSlope/normFactorTypVentlateFlatness=rootMeanSqFlowPredErr/normFactorTypVent

“Flatness” here refers to the extent to which the late section resemblesa straight line, with no implications about its gradient, and the valueof lateFlatness is zero when the late section is actually a straightline.

At the next step 7460, the method 7400 computes a quantityflowAboveLateBeforeLate that indicates by how much the inspiratorywaveform before the late section exceeds the back projection of thelinear approximation to the late section of the inspiratory waveformcomputed at step 7455, i.e. indicates the size of an early peak in theinspiratory waveform. To compute flowAboveLateBeforeLate, the earliestlocation before the late section at which the respiratory flow rate Qrexceeds the back projection of the linear approximation to the latesection is found. The difference between the respiratory flow rate Qrand the linear approximation is then averaged between this location andthe start location of the late section. The average is then normalisedby the normalising factor normFactorTypVent to obtainflowAboveLateBeforeLate. Step 7460 then computes a fuzzy truth variableflowAboveLateBeforeLateAdequate from the value offlowAboveLateBeforeLate such that flowAboveLateBeforeLateAdequate isfuzzy false until flow AboveLateBeforeLate exceeds a threshold thatgenerally increases with the mean flow rate of the late sectionlateFlowMean (which was computed at step 7455). The effect of thedependence of flowAboveLateBeforeLateAdequate on lateFlowMean is thatlarger early peaks are required to cause fuzzyLateFlatness to be fuzzytrue as the mean flow rate of the late section increases.

In one implementation, step 7460 computesflowAboveLateBeforeLateAdequate as follows:

flowAboveLateBeforeLateAdequate = FuzzyMember(          flowAboveLateBeforeLate,           LOWER_THRESH, False,          UPPER_THRESH, True               )

where LOWER_THRESH and UPPER_THRESH are constant thresholds equal in oneimplementation to 0.5*lateFlowMean and lateFlowMean respectively. In oneimplementation, the thresholds have minimum values of 2.25 and 4.5litres/minute respectively, regardless of the value of lateFlowMean.

The method 7400 proceeds to step 7465, which computes a fuzzy truthvariable lateFlatnessFuzzyComp which turns from true to false aslateFlatness increases, i.e. the late section becomes less linear. Inone implementation, step 7465 computes lateFlatnessFuzzyComp as follows:

lateFlatnessFuzzyComp = FuzzyMember(       lateFlatness,      LOWER_FLATNESS_THRESH, True,       UPPER_FLATNESS_THRESH, False         )

where LOWER_FLATNESS_THRESH and UPPER_FLATNESS_THRESH are constantthresholds equal in one implementation to 0.6 and 0.9 respectively.

The next step 7470 computes a fuzzy truth variable lateSlopeFuzzyCompthat is only true when lateSlope is “moderate”, i.e. within a certainrange. That is, large positive or negative values of lateSlope may causelateSlopeFuzzyComp to be false. In one implementation, step 7470computes lateSlopeFuzzyComp as follows:

lateSlopeFuzzyComp = FuzzyMember(         lateSlope,      LOWER_SLOPE_THRESH, False,       UPPER_SLOPE_THRESH, True,          6.0, True,           9.0, False,             )

where LOWER_SLOPE_THRESH and UPPER_SLOPE_THRESH are thresholds. In oneimplementation, the thresholds generally become more negative aslateFlowMean increases, meaning that larger negative slopes arepermitted within the definition of “moderate” as the mean flow rate ofthe late section increases. In one implementation, step 7470 computesLOWER_SLOPE_THRESH and UPPER_SLOPE_THRESH as follows:LOWER_SLOPE_THRESH=−9.0−3*slopeThresholdExtensionUPPER_SLOPE_THRESH=−6.0−3*slopeThresholdExtension

where slopeThresholdExtension is a real number that decreases from 1 to0 as lateFlowMean increases. In one implementation, step 7470 computesslopeThresholdExtension as follows:

slopeThresholdExtension = FuzzyDeweight(           lateFlowMean,            0.6, 1.0,             0.9, 0.0             )

The final step 7475 of the method 7400 computes fuzzyLateFlatness as thefuzzy AND of the fuzzy truth variables flowAboveLateBeforeLateAdequate,lateFlatnessComp, and lateSlopeFuzzyComp computed at steps 7460, 7465,and 7470 respectively.

7.4.3.2.5 M-Shape Detection

In one form of the present technology, the therapy engine 4320 moduleexecutes one or more algorithms to detect “M-shape” in the inspiratorywaveform. In one form, the M-shape detection algorithm 4326 receives asan input a respiratory flow rate signal Qr and provides as an output ameasure indicative of the extent to which each inspiratory waveformexhibits M-shape.

M-shaped inspiratory waveforms with tidal volumes or other breathwiseventilation values not much greater than typical recent values areindicative of flow limitation. Such inspiratory waveforms have arelatively rapid rise and fall and a dip or “notch” in flowapproximately in the centre, the dip being due to flow limitation (seeFIGS. 6E and 6F). At higher tidal volumes or breathwise ventilationvalues, such waveforms are generally behavioural, i.e. micro-arousalsduring sleep, or sighs, and are not indicative of flow limitation.

To detect M-shaped waveforms, the M-shape detection algorithm 4326determines the similarity of the inspiratory waveform to a waveformwhich is broadly M-shaped.

FIG. 7F is a flow chart illustrating a method 7500 that may be used toimplement the M-shape detection algorithm 4326 in one form of thepresent technology.

Since the notch may not be at the centre of the inspiratory waveform,the method 7500 attempts to find the location of the notch, and thenlinearly time-distorts the waveform so that the notch is at the centreof the waveform. To find the notch, the first step 7510 performs amodified convolution of the normalised inspiratory waveform f(t)(wherein the normalisation is division by the mean) with a V-shapedkernel V(t) of length Ti/2, centred on zero, where Ti is the inspiratorytime:

${V(t)} = {{8{\frac{t}{T_{i}}}} - 1}$

The modified convolution is based on separate convolutions with the leftand right halves of the kernel V(t). The left half convolution iscalculated as

${I_{L}(\tau)} = {\overset{0}{\int\limits_{\frac{- T_{i}}{4}}}{{V(t)}{f\left( {t - \tau} \right)}{dt}}}$

and the right half convolution as

${I_{R}(\tau)} = {\overset{\frac{- T_{i}}{4}}{\int\limits_{0}}{{V(t)}{f\left( {t - \tau} \right)}{dt}}}$

The modified convolution I(τ) is computed as a combination of the leftand right half convolutions I_(L)(τ) and I_(R)(τ) such that if either ofthe left and right half convolutions is zero, the result is zero,regardless of the other quantity, and if both are 1, the result is 1.Thus constrained, the combination of the left and right halfconvolutions resembles a logical “and” function in some sense, hence isgiven the name “V-anded convolution”. In one implementation, thecombination is a modified geometric mean of the left and right halfconvolutions:

$\begin{matrix}{{I(\tau)} = \left\{ \begin{matrix}\sqrt{{{I_{L}(\tau)}{I_{R}(\tau)}},} & {{I_{L}(\tau)} > {0\mspace{14mu}{and}\mspace{14mu}{I_{R}(\tau)}} > 0} \\0 & {otherwise}\end{matrix} \right.} & (1)\end{matrix}$

The above constraint provides a condition that the inspiratory waveformto the left of the posited notch is generally increasing leftwards, andthat to the right of the notch is generally increasing rightwards. Thisprovides more specificity than simply summing the left and rightintegrals. In the implementation given in equation (1), the integrals ofthe product of the time-shifted normalised inspiratory waveform witheach half-V must be strictly positive, otherwise the V-anded convolutionis zero. This prevents a variety of pathologies, for example, when thepart of the inspiratory flow to the left of the centre of the V does notactually increase leftwards, but the integral of the right half of the Vwaveform is so large that it overwhelms an actually decreasing lefthalf.

The V-anded convolution is performed with the position of the centre ofthe kernel V(t) ranging from Ti/4 to 3 Ti/4, thus yielding results forthe central half of the inspiratory waveform.

Step 7520 finds the location at which the modified convolution I(τ)peaks, and if the height of this peak is greater than a threshold, anotch is deemed to exist at the location t_(notch) of the centre of thekernel V(t) at which this peak is located. In one implementation, thethreshold is set to 0.15.

If a notch is found by step 7520 (“Y”) at the location t_(notch), theinspiratory waveform f(t) is then, at step 7530, time distorted or“symmetrised” so that half the waveform is to the left of t_(notch) andhalf is to the right. This operation gives a time-distorted or“symmetrised” version G(t) of the inspiratory waveform f(t):

${G(t)} = \left\{ \begin{matrix}{{f\left( {\frac{t}{\left( \frac{T}{2} \right)}t_{notch}} \right)},} & {t < \frac{T}{2}} \\{f\left( {t_{notch} + {\left( \frac{t - \frac{T}{2}}{\frac{T}{2}} \right)\left( {T - t_{notch}} \right)}} \right)} & {t \geq \frac{T}{2}}\end{matrix} \right.$

If no notch is found at step 7520 (“N”), step 7535 sets G(t) to theinspiratory waveform f(t), since some waveforms that do not exhibit adetectable notch may still have M-shaped flow limitation.

To detect M-shaped flow limitation in the symmetrised waveform G(t),first and third sinusoidal harmonic functions of half-width Ti are firstdefined as

${F_{1}(t)} = {\sin\left( {\pi\frac{t}{Ti}} \right)}$ and${F_{3}(t)} = {\sin\left( {3\pi\frac{t}{Ti}} \right)}$

These two harmonic functions are orthogonal on [0, Ti]. For t in [0,Ti], F₃(t) is broadly similar to an M-shaped inspiratory waveform, andF₁(t) is broadly similar to a normal inspiratory waveform. Hence theextent to which the symmetrised waveform G(t) resembles F₃(t) is anindicator of how much the waveform resembles an M. Step 7540 calculatesthis extent. In one implementation, step 7540 calculates the power inthe first harmonic of the symmetrised waveform G(t) as the square of theinner product of the first harmonic function F₁ with the symmetrisedwaveform G(t), and the power in the third harmonic of the symmetrisedwaveform G(t) as the square of the inner product of the third harmonicfunction F₃ with the symmetrised waveform G(t). Both inner products arecalculated over the inspiratory interval [0, Ti]. Step 7540 thencomputes the extent to which the symmetrised waveform G(t) resemblesF₃(t) as the ratio M3Ratio of the power in the third harmonic of thesymmetrised waveform G(t) to the sum of the powers in the first andthird harmonics of the symmetrised waveform G(t):

${M3Ratio} = \frac{\left\langle {F_{3},G} \right\rangle_{\lbrack{0,{Ti}}\rbrack}^{2}}{\left\langle {F_{1},G} \right\rangle_{\lbrack{0,{Ti}}\rbrack}^{2} + \left\langle {F_{3},G} \right\rangle_{\lbrack{0,{Ti}}\rbrack}^{2}}$

When M3Ratio is large, the inspiratory waveform typically resembles anM. But M3Ratio can also be large if the waveform is very asymmetric,with a much higher mean flow in either the first or second half of thewaveform than in the other half. To exclude this possibility, step 7540also calculates a measure Symm of the symmetry of the inspiratorywaveform f(t) about the notch location. In one implementation, step 7540calculates the third harmonic components of the first and second halvesof the symmetrised waveform G(t):

$M_{3L} = \left\langle {F_{3},G} \right\rangle_{\lbrack{0,\frac{Ti}{2}}\rbrack}$$M_{3R} = \left\langle {F_{3},G} \right\rangle_{\lbrack{\frac{Ti}{2},{Ti}}\rbrack}$

Step 7540 then calculates the measure Symm as the ratio of the lesser ofthese components to the sum of their absolute values:

${Symm} = \frac{\min\left( {M_{3L},M_{3R}} \right)}{{M_{3L}} + {M_{3R}}}$

Step 7550 then tests whether the measure Symm is less than a lowthreshold, set in one implementation to 0.3. If so (“Y”), theinspiratory waveform is deemed not to be symmetrically M-shaped, and aquantity M3SymmetryRatio, which is a measure of the extent to which theinspiratory waveform is symmetrically M-shaped, is set equal to zero atstep 7560. Otherwise (“N”), M3SymmetryRatio is set equal to M3Ratio atstep 7570.

The final step 7580 computes a variable RxProportion fromM3SymmetryRatio so as to generally increase from 0 to 1 asM3SymmetryRatio increases. In one implementation, step 7580 computesRxProportion as follows:

RxProportion = FuzzyDeweight(       M3SymmetryRatio,  LOWER_M3SYMMETRYRATIO_THRESH, 0.0,   UPPER_M3SYMMETRYRATIO_THRESH, 1.0             )

where LOWER_M3SYMMETRYRATIO_THRESH and UPPER_M3SYMMETRYRATIO_THRESH areconstants, equal in one implementation to 0.17 and 0.3 respectively. Thevariable RxProportion is an indicator in the range [0, 1] of the degreeof M-shape of the inspiratory portion of the respiratory flow ratewaveform.

7.4.3.2.6 Apnea Detection

In one form of the present technology, the therapy engine module 4320executes an apnea detection algorithm 4325 to detect apneas.

In one form, the apnea detection algorithm 4325 receives as an input arespiratory flow rate signal Qr and provides as an output a series ofevents indicating starts and ends of detected apneas.

FIG. 7G is a flow chart illustrating a method 7600 of detecting apneasthat may be used to implement the apnea detection algorithm 4325 in oneform of the present technology.

The method 7600 generally looks for low ventilation in relation to anexpected normal ventilation Vnorm. In some implementations, the expectednormal ventilation Vnorm may be the typical recent ventilation Vtypreturned by the typical recent ventilation determination algorithm 4328,described below.

Ventilation is measured at two different timescales, one comparable to abreath and one of several breaths' duration, and low ventilation inrelation to the expected normal ventilation Vnorm at either timescaleindicates an apnea is in progress. The method 7600 contains hysteresis,in that if an apnea has previously been determined to be in progress butneither of the two criteria that indicate an apnea is in progress istrue, a further criterion derived from the ventilation relative toexpected normal ventilation Vnorm must be true for the apnea to haveended. The hysteresis of the method 7600 gives the method 7600 greaterrobustness to transient increases in respiratory flow rate than previousmethods of detecting apneas.

The method 7600 also returns an effective duration for the apnea fortreatment purposes. The effective duration of the apnea is by defaultthe total duration of its closed-airway portions, that is, the sum ofthe elapsed time between the commencement of each closed-airway portionof the apnea and its termination. However, the effective duration of theapnea may be reduced from this default “closed duration” value by eitheror both of two separate deweighting factors, one derived from theventilation relative to the expected normal ventilation Vnorm, and onederived from the estimated leak flow rate Ql during the apnea. Theeffect of this deweighting is that to the extent that leak is large,and/or the ventilation approaches the expected normal ventilation Vnormduring the apnea, the effective duration of the apnea is shorter thanits closed duration, so the prescribed EPAP increase is less.

The method 7600 starts at step 7610, which computes a short-timeventilation Vshort as the mean over the most recent “short interval” ofthe instantaneous ventilation Vint returned by the ventilationdetermination algorithm 4323. In one implementation, the short intervalis two seconds. Step 7610 also computes a “very fast” relativeventilation error veryFastRelVentError as a relative difference betweenthe “very fast ventilation” VveryFast obtained from the ventilationdetermination algorithm 4323 and the expected normal ventilation Vnorm.Step 7610 computes the very fast relative ventilation errorveryFastRelVentError as follows:veryFastRelVentErr=(VveryFast−Vnorm)/Vnorm

The method 7600 then proceeds to check at step 7615 whether theshort-time ventilation Vshort is less than or equal to than a lowfraction of the expected normal ventilation Vnorm. In one implementationthe low fraction is set to 0.2. If so (“Y”), an apnea is detected. Ifnot (“N”), the next step 7620 checks whether veryFastRelVentError isless than or equal to an apnea threshold, equal in one implementation to−0.95. The check at step 7620 is equivalent to determining whether thevery fast ventilation VveryFast is less than a small fraction of theexpected normal ventilation Vnorm, the small fraction in this case being0.05. If so (“Y”), an apnea is detected.

The method 7600 maintains the current state of apnea as a Booleanindicator inApnea (which is initialised to False at the start oftherapy). If either apnea start criterion (tested in steps 7615 and7620) is satisfied (“Y”), the method 7600 checks at step 7630 whether anapnea was already in progress by checking whether inApnea is True. Ifnot (“N”), the next step 7635 issues an Apnea Start event and setsinApnea to True to indicate that the patient 1000 has just entered anapnea state. Step 7645 then sets the effective duration of the apnea tozero. The effective duration of the apnea is stored in a variableeffectiveApneaDuration that is maintained as long as the apnea statepersists.

The next step 7670 sets a variable called intVentErrAboveThresh to zero.This variable, which holds a running total of the amount by which thevery fast relative ventilation error veryFastRelVentError computed atstep 7610 exceeds the apnea threshold, is used to confirm that an apneahas indeed ended if neither of the two apnea start criteria tested at7615 and 7620 is satisfied.

If the check at step 7630 finds that the patient 1000 is alreadyexperiencing an apnea (“Y”), the method 7600 proceeds directly to step7670 to set the running total intVentErrAboveThresh to zero as describedabove. The method 7600 then proceeds to step 7680 described below.

If neither of the apnea start criteria are satisfied (“N” at steps 7615and 7620), the method 7600 checks whether an apnea is already inprogress by checking at step 7640 whether inApnea is True. If not (“N”),the method 7600 concludes at step 7655. Otherwise (“Y”), it is possiblethat the current apnea has ended. However, as mentioned above, themethod 7600 confirms this by proceeding to step 7650 which updates therunning total intVentErrAboveThresh by adding the current differencebetween veryFastRelVentError and the apnea threshold, provided thatdifference is greater than zero.

Step 7660 then checks an apnea end criterion, namely whether the runningtotal intVentErrAboveThresh is greater than a threshold. In oneimplementation, this threshold is set to 0.15. If the apnea endcriterion is satisfied (“Y”), the apnea is ended, so step 7665 ends theapnea state by issuing an Apnea End event and sets inApnea to false. Themethod 7600 then concludes.

If the apnea end criterion is not satisfied, i.e. the running totalintVentErrAboveThresh is not greater than the threshold (“N” at step7660), the apnea is still in progress, and the method 7600 proceeds tostep 7680 described below.

Step 7680 computes a deweighting factor apneaTimeWeighting fromveryFastRelVentError such that apneaTimeWeighting decreases from 1 to 0as veryFastRelVentError increases above the apnea threshold. In oneimplementation, step 7680 computes apneaTimeWeighting as follows:

apneaTimeWeighting = FuzzyDeweight(       veryFastRelVentError,      APNEA_THRESHOLD, 1.0,   APNEA_THRESHOLD_PLUS_A_BIT, 0.0           )

where APNEA_THRESHOLD is the apnea threshold of step 7620 andAPNEA_THRESHOLD_PLUS_A_BIT is a threshold that is set slightly higherthan APNEA_THRESHOLD, equal in one implementation toAPNEA_THRESHOLD+0.05. The deweighting factor apneaTimeWeightingdiscounts the contribution of the current time instant to the finaleffective duration of the apnea.

Step 7685 then computes a second deweighting factor leakDeweighting,based on the leak flow rate estimate Ql from the leak flow rateestimation algorithm 4316. In one implementation, step 7685 computesLeakDeweighting such that as the estimated leak flow rate increases,LeakDeweighting decreases from 1 to 0.

LeakDeweighting = FuzzyDeweight(           Ql,       LOWER_LEAK_THRESH,1.0,       UPPER_LEAK_THRESH, 0.0           )

where LOWER_LEAK_THRESH and UPPER_LEAK_THRESH are constants, equal inone implementation to 48 litres/minute and 60 litres/minuterespectively.

Finally, at step 7690, the method 7600 updates the effective duration ofthe apnea effectiveApneaDuration taking into account the two deweightingfactors apneaTimeWeighting and LeakDeweighting and the airway patencydetermined by the airway patency determination algorithm 4327 (describedbelow).

In one implementation, step 7690 updates the effective duration of theapnea by incrementing the current value of effectiveApneaDuration by theproduct of the two deweighting factors apneaTimeWeighting andLeakDeweighting, if the airway is currently determined to be closed.Otherwise, step 7690 does not update the effective duration of theapnea.

The final value of effectiveApneaDuration returned by the apneadetection algorithm 4325 for a completed apnea will be in units of timeof duration equal to the apnea detection interval. The apnea detectioninterval is the reciprocal of the frequency at which the apnea detectionalgorithm 4325 is executed by the therapy engine module 4320.

7.4.3.2.7 Typical Recent Ventilation Determination

In one form of the present technology, the central controller 4230 takesas input the measure of current ventilation, Vent, and executes one ormore typical recent ventilation determination algorithms 4328 for thedetermination of a value Vtyp indicative of the typical recentventilation of the patient 1000.

The typical recent ventilation Vtyp is the value around which thedistribution of the measure of current ventilation Vent over multipletime instants over some predetermined timescale tends to cluster, thatis, a measure of the central tendency of the measure of currentventilation over recent history. In one implementation of the typicalrecent ventilation determination algorithm 4328, the recent history isof the order of several minutes, but in any case should be longer thanthe timescale of Cheyne-Stokes waxing and waning cycles. The typicalrecent ventilation determination algorithm 4328 may use any of thevariety of well-known measures of central tendency to determine thetypical recent ventilation Vtyp from the measure of current ventilation,Vent. One such measure is the output of a low-pass filter on the measureof current ventilation Vent, with time constant equal to one hundredseconds.

7.4.3.2.8 Airway Patency Determination

In one form of the present technology, the central controller 4230executes an airway patency determination algorithm 4327 for thedetermination of airway patency. In some implementations, the airwaypatency determination algorithm 4327 returns either “closed” or “open”,or equivalent Boolean values, e.g. “true” indicating closed and “false”indicating open.

As described above, in the absence of spontaneous triggering, the RPTdevice 4000 delivers backup breaths at the backup rate Rb. Backupbreaths delivered during an apnea state as detected by the apneadetection algorithm 4325 are referred to as “probe breaths”.

Probe breaths may have the same amplitude as normal backup breaths (thepressure support value A), but may have their own timing parameters,i.e. special values of Timin and Timax referred to as Timin_PB andTimax_PB. The values of Timin_PB and Timax_PB may be derived from othersettings of the RPT device 4000. In one implementation, Timin_PB andTimax_PB are defined asTimin_PB=Max(0.1,Min(1.2,0.4*Tbackup)  (2)Timax_PB=Min(Max(Timax,Timin_PB),0.4*Tbackup)  (3)

The main reason for modifying Timin for probe breaths is to avoid theprobe breath being early cycled such that the probe breath durationwould become too short to provide reliable information relative to thestate of the upper airway.

In some implementations, the airway patency determination algorithm 4327is not invoked when the pressure support A (see below) of the therapy isabove a threshold, for example 10 cmH₂O. For pressure support A abovethis threshold, any apnea is deemed to be a closed airway apnea.

The airway patency determination algorithm 4327 analyses the respiratoryflow rate Qr in response to the probe breaths delivered while the RPTdevice 4000 is in an apnea state to determine the patency of the airwayduring the apnea. The opened/closed airway determination is based onanalysing the shape of the inspiratory and expiratory portions of therespiratory flow rate signal Qr in response to the probe breaths.Generally speaking, a “decreasing” portion is indicative of an openairway, while a “plateauing” portion is indicative of a closed airway.

FIGS. 7H and 7I contain a flow chart illustrating a method 7700 that maybe used to implement the airway patency determination algorithm 4327 inone form of the present technology. The method 7700 takes as inputs theinspiratory portion of the respiratory flow rate waveform (theinspiratory waveform) and the expiratory portion of the respiratory flowrate waveform (the expiratory waveform) in response to a probe breath.The method 7700 starts at step 7710, which “trims” the inspiratory andexpiratory waveforms. The purpose of trimming is to remove the “initialrush” of flow as gaseous compression and compliant elements in theairpath take up some volume. In one implementation, step 7710 trims theinspiratory waveform to the “late section” of the inspiratory waveformas defined above in relation to the “chairness” detection step 7030 ofthe method 7000. In such an implementation, the start location of thelate section is 1.25 multiplied by the rise time, and the end locationof the late section is one quarter of the fall time back from the end ofthe inspiratory waveform. The expiratory portion may be trimmed insimilar fashion, interchanging rise time and fall time.

Step 7720 follows, at which linear regression is performed on thetrimmed inspiratory and expiratory waveforms. Linear regression findsthe “best” (in a least-squares sense) straight-line approximation to afunction over an interval, returning the following parameters: gradient,y-intercept (initial value), mean value, and correlation coefficient(which indicates the “goodness of fit” of the straight-lineapproximation). At the next step 7730, the method 7700 computes a meanvalue Qmean of the absolute value of the respiratory flow rate Qr overthe trimmed inspiratory and expiratory waveforms. Step 7730 then dividesthis value Qmean by the typical recent ventilation Vtyp calculated bythe typical recent ventilation determination algorithm 4328 to obtain anvalue Sz indicative of the size of the response to the probe breath inrelation to the level of current ventilation. Step 7740 then determinesan upper threshold and a lower threshold with which the breath sizeindicator value Sz is to be compared. In some forms of step 7740, thelower threshold is a function of the current pressure support A (seebelow), which is the amplitude of the delivered probe breath. In onesuch form, the lower threshold is the value of A clipped to the range [4cmH₂O, 8 cmH₂O] and divided by 20 cmH₂O. The lower threshold willtherefore range from 0.2 to 0.4 under such an implementation. In oneimplementation of step 7740, the upper threshold is set to 0.75.

Step 7750 then compares the breath size indicator value Sz computed atstep 7730 with the lower threshold. If the breath size indicator valueSz is below the lower threshold (“Y”), the method 7700 concludes at step7770 by returning the value “closed” as the respiratory flow rate sizeis too small for the airway to be deemed open. Otherwise (“N”), themethod 7700 proceeds to step 7760, which determines whether the breathsize indicator value Sz is above the upper threshold. If so (“Y”), themethod 7700 concludes at step 7770 by returning the value “open” as therespiratory flow rate size is too large for the airway to be deemedclosed. If not (“N”), the method 7700 proceeds to step 7790, whichcomputes a value Sz_rel for the breath size indicator value Sz relativeto the lower and upper thresholds as follows:Sz_rel=(Sz−lower_threshold)/(upper_threshold−lower_threshold)

The relative breath size indicator value Sz_rel ranges from 0 (when thebreath size indicator value Sz is equal to the lower threshold) to 1(when the breath size indicator value Sz is equal to the upperthreshold).

The method 7700 then continues from step 7810 in FIG. 7I for a finaldetermination of the airway patency based on the relative breath sizeindicator value Sz_rel and the straight-line approximations to thetrimmed inspiratory and expiratory waveforms found in step 7720 (whichmay in some implementations take place after the steps 7730 to 7790).

In step 7810, the method 7700 computes the “final value” of thestraight-line approximation to the trimmed inspiratory waveform. Thefinal value may be computed by adding the y-intercept (initial value) tothe product of the gradient and the duration of the trimmed inspiratoryportion. Step 7810 then computes an “inspiratory flatness ratio” as theratio of the final value to the initial value of the straight-lineapproximation to the trimmed inspiratory waveform.

In the next step 7820, the method 7700 computes the “final value” of thestraight-line approximation to the trimmed expiratory waveform. Thefinal value may be computed by adding the y-intercept (initial value) tothe product of the gradient and the duration of the trimmed expiratoryportion. Step 7820 then computes an “expiratory flatness ratio” as theratio of the final value to the initial value of the straight-lineapproximation to the trimmed expiratory waveform.

Step 7830 follows, at which the method 7700 computes inspiratory andexpiratory thresholds with which the inspiratory and expiratory flatnessratios respectively are to be compared. If either ratio falls below itscorresponding threshold, the airway is determined to be open; otherwise,the airway is determined to be closed.

In one implementation of step 7830, the thresholds may be computed so asto be linearly increasing as the relative breath size indicator valueSz_rel increases from 0 to 1. For example,inspiratory_threshold=Sz_rel  (4)expiratory_threshold=Sz_rel  (5)

The effect of this is that for probe breaths whose size indicator valueSz is large enough to be close to the upper threshold above which theairway is determined to be open in step 7780, the inspiratory andexpiratory thresholds will be close to 1, so the inspiratory andexpiratory flatness ratios are more likely to be below threshold, andthe airway to be determined to be open. Conversely, for probe breathswhose size indicator value Sz is small enough to be close to the lowerthreshold below which the airway is determined to be closed in step7770, the inspiratory and expiratory thresholds will be close to 0, sothe inspiratory and expiratory flatness ratios are more likely to beabove threshold, and the airway determined to be closed.

In some implementations, this latter determination may qualified byadjusting the inspiratory and expiratory thresholds based on the“goodness of fit” of the straight-line approximations to the inspiratoryand expiratory waveforms. To the extent that the straight-lineapproximation is a “good fit”, as quantified by the correlationcoefficient, the lowest value of the threshold is proportionally greaterthan zero, making an “open” determination more likely even forrelatively small breaths. In one such implementation, a “goodness offit” parameter is computed for the inspiratory waveform asinsp_goodness=InterpOnInterval(Insp_corrcoefft,−0.95,0.5,−0.8,0.0)

which is equal to 0.5 when the inspiratory correlation coefficient isbelow −0.95 (indicating a good fit), equal to 0 when the inspiratorycorrelation coefficient is above −0.8, and linearly decreasing from 0.5to 0 when the inspiratory correlation coefficient is above −0.95 andbelow −0.8.

In this implementation, a “goodness of fit” parameter is computed forthe expiratory waveform asexp_goodness=InterpOnInterval(Exp_corrcoefft,0.8,0,0.95,0.5)

which is equal to 0.5 when the expiratory correlation coefficient isabove 0.95 (indicating a good fit), equal to 0 when the expiratorycorrelation coefficient is below 0.8, and linearly increasing from 0 to0.5 when the expiratory correlation coefficient is above 0.8 and below0.95.

In this implementation, equations (4) and (5) for the inspiratory andexpiratory thresholds are modified as follows:inspiratory_threshold=insp_goodness+(1−insp_goodness)*Sz_relexpiratory_threshold=exp_goodness+(1−exp_goodness)*Sz_rel

In this implementation, the inspiratory and expiratory thresholdslinearly increase from 0 to 1 as Sz_rel increases from 0 to 1 when the“goodness of fit” is poor, but when it is good, the thresholds linearlyincrease from 0.5 to 1 as Sz_rel increases from 0 to 1. Thus for anygiven value of Sz_rel, the better the straight-line fit, the higher arethe thresholds, increasing the chance that the airway will be found tobe open for reasonably “linear” breaths.

Returning to the method 7700, at step 7840 the inspiratory flatnessratio is compared with the inspiratory threshold. If the inspiratoryflatness ratio is below the inspiratory threshold (“Y”), at step 7870the airway is determined to be open and the method 7700 concludes.Otherwise (“N”), at step 7850 the expiratory flatness ratio is comparedwith the expiratory threshold. If the expiratory flatness ratio is belowthe expiratory threshold (“Y”), at step 7870 the airway is determined tobe open and the method 7700 concludes. Otherwise (“N”), at step 7860 theairway is determined to be closed and the method 7700 concludes.

7.4.3.2.9 Determination of Therapy Parameters

In some forms of the present technology, the central controller 4230executes one or more therapy parameter determination algorithms 4329 forthe determination of one or more therapy parameters using the valuesreturned by one or more of the other algorithms in the therapy enginemodule 4320.

In one form of the present technology, the therapy parameter is aninstantaneous treatment pressure Pt. In one implementation of this form,the therapy parameter determination algorithm 4329 determines thetreatment pressure Pt as follows:Pt=AΠ(Φ,t)+P ₀  (6)where A is the amount of “pressure support”, Π(Φ, t) is the waveformtemplate value (in the range 0 to 1) at the current values Φ of phaseand t of time, and P₀ is a base pressure.

By determining the treatment pressure Pt using equation (6) and applyingit as a set point in the controller 4230 of the RPT device 4000, thetherapy parameter determination algorithm 4329 oscillates the treatmentpressure Pt in synchrony with the spontaneous respiratory effort of thepatient 1000. That is, based on the typical waveform templates Π(Φ)described above, the therapy parameter determination algorithm 4329increases the treatment pressure Pt at the start of, or during, orinspiration and decreases the treatment pressure Pt at the start of, orduring, expiration. The (non-negative) pressure support A is theamplitude of the oscillation.

If the waveform determination algorithm 4322 provides the waveformtemplate Π(Φ) as a lookup table, the therapy parameter determinationalgorithm 4329 applies equation (6) by locating the nearest lookup tableentry to the current value Φ of phase returned by the phasedetermination algorithm 4321, or by interpolation between the twoentries straddling the current value Φ of phase.

The values of the pressure support A and the base pressure P₀ may bedetermined by the therapy parameter determination algorithm 4329depending on the chosen respiratory pressure therapy mode in the mannerdescribed below.

7.4.3.3 Therapy Control Module

Therapy control module 4330 in accordance with one aspect of the presenttechnology receives as inputs the therapy parameters from the therapyparameter determination algorithm 4329 of the therapy engine module4320, and controls the pressure generator 4140 to deliver a flow of airin accordance with the therapy parameters.

In one form of the present technology, the therapy parameter is atreatment pressure Pt, and the therapy control module 4330 controls thepressure generator 4140 to deliver a flow of air whose mask pressure Pmat the patient interface 3000 is equal to the treatment pressure Pt.

7.4.3.4 Detection of Fault Conditions

In one form of the present technology, the central controller 4230executes one or more methods for the detection of fault conditions. Thefault conditions detected by the one or more methods may include atleast one of the following:

-   -   Power failure (no power, or insufficient power)    -   Transducer fault detection    -   Failure to detect the presence of a component    -   Operating parameters outside recommended ranges (e.g. pressure,        flow rate, temperature, PaO₂)    -   Failure of a test alarm to generate a detectable alarm signal.

Upon detection of the fault condition, the corresponding algorithmsignals the presence of the fault by one or more of the following:

-   -   Initiation of an audible, visual &/or kinetic (e.g. vibrating)        alarm    -   Sending a message to an external device    -   Logging of the incident

7.5 Humidifier

In one form of the present technology there is provided a humidifier5000 (e.g. as shown in FIG. 5A) to change the absolute humidity of airor gas for delivery to a patient relative to ambient air. Typically, thehumidifier 5000 is used to increase the absolute humidity and increasethe temperature of the flow of air (relative to ambient air) beforedelivery to the patient's airways.

The humidifier 5000 may comprise a humidifier reservoir 5110, ahumidifier inlet 5002 to receive a flow of air, and a humidifier outlet5004 to deliver a humidified flow of air. In some forms, as shown inFIG. 5A and FIG. 5B, an inlet and an outlet of the humidifier reservoir5110 may be the humidifier inlet 5002 and the humidifier outlet 5004respectively. The humidifier 5000 may further comprise a humidifier base5006, which may be adapted to receive the humidifier reservoir 5110 andcomprise a heating element 5240.

7.6 Respiratory Pressure Therapy Modes

Various respiratory pressure therapy modes may be implemented by the RPTdevice 4000 depending on the values of the parameters A and P₀ in thetreatment pressure equation (6) used by the therapy parameterdetermination algorithm 4329 in one form of the present technology.

7.6.1 CPAP Therapy

In some implementations, the pressure support A is identically zero, sothe treatment pressure Pt is identically equal to the base pressure P₀throughout the respiratory cycle. Such implementations are generallygrouped under the heading of CPAP therapy. In such implementations,there is no need for the therapy engine module 4320 to determine phase Φor the waveform template Π(Φ).

7.6.2 Ventilation Therapy

In other implementations, the value of pressure support A in equation(6) may be positive. Such implementations are known as ventilationtherapy. In some forms of ventilation therapy, known as fixed pressuresupport ventilation therapy, the pressure support A is fixed at apredetermined value, e.g. 10 cmH₂O. The predetermined value of pressuresupport A is a setting of the RPT device 4000, and may be set forexample by hard-coding during configuration of the RPT device 4000 or bymanual entry through the input device 4220.

The value of the pressure support A may be limited to a range defined as[Amin, Amax]. The pressure support limits Amin and Amax are settings ofthe RPT device 4000, set for example by hard-coding during configurationof the RPT device 4000 or by manual entry through the input device 4220.A minimum pressure support Amin of 3 cmH₂O is of the order of 50% of thepressure support required to perform all the work of breathing of atypical patient in the steady state. A maximum pressure support Amax of12 cmH₂O is approximately double the pressure support required toperform all the work of breathing of a typical patient, and thereforesufficient to support the patient's breathing if they cease making anyefforts, but less than a value that would be uncomfortable or dangerous.

7.6.2.1 Auto-Titration of the EPAP

In ventilation therapy modes, the base pressure P₀ is sometimes referredto as the EPAP. The EPAP may be a constant value that is prescribed ordetermined via a process known as titration. Such a constant EPAP may beset for example by hard-coding during configuration of the RPT device4000 or by manual entry through the input device 4220. This alternativeis sometimes referred to as fixed-EPAP ventilation therapy. Titration ofthe constant EPAP for a given patient may be performed by a clinicianwith the aid of a PSG study carried out during a titration session.

Alternatively, the therapy parameter determination algorithm 4329 mayrepeatedly compute the EPAP during ventilation therapy. In suchimplementations, the therapy parameter determination algorithm 4329repeatedly computes the EPAP as a function of indices or measures ofupper airway instability returned by the respective algorithms in thetherapy engine module 4320, such as one or more of inspiratory flowlimitation and apnea. Because the repeated computation of the EPAPresembles the manual adjustment of the EPAP by a clinician duringtitration of the EPAP, this process is also sometimes referred to asauto-titration of the EPAP, and the overall therapy is known asauto-titrating EPAP ventilation therapy, or auto-EPAP ventilationtherapy.

The role of airway patency is particularly important in auto-EPAPventilation therapy. When the pressure support A is greater than somethreshold (e.g. 10 cmH₂O), if the apnea detection algorithm 4325 detectsan apnea, it may be inferred that the airway is closed, since otherwisethe pressure support A would be large enough to generate some sort ofrespiratory airflow and an apnea would not have been detected.

However, when the pressure support A is less than the threshold, if theapnea detection algorithm 4325 detects an apnea, the RPT device 4000cannot immediately infer that the airway is closed. The airway patencydetermination algorithm 4327 therefore repeatedly determines a measureof airway patency while the RPT device 4000 is in an apnea state. Whilethe apnea state persists, the apnea detection algorithm 4325 accumulatesthe duration over which the upper airway is determined to be closed.This accumulated duration is defined as the effective duration of theapnea.

At the conclusion of the apnea state, the RPT device 4000 determines anEPAP increment (prescription) from the effective duration of the apnea.The longer is the effective duration of the apnea, the greater is theincrement.

FIG. 8A is a flow chart illustrating a method 8000 of auto-titrating theEPAP suitable for use in conjunction with non-invasive ventilationtherapy. The method 8000 may be repeatedly implemented as part of thetherapy parameter determination algorithm 4329.

The method 8000 auto-titrates the EPAP by maintaining and updating a“desired” EPAP value, which is a target value or set point towards whichthe actual EPAP is repeatedly adjusted. The desired EPAP is updateddepending on the extent of inspiratory flow limitation determined by theinspiratory flow limitation determination algorithm 4324, M shapedetermined by the M-shape detection algorithm 4326, recent apneasdetected by the apnea detection algorithm 4325, and the current EPAP.

The method 8000 auto-titrates the EPAP within a range [EPAPmin,EPAPmax]. The EPAP lower and upper limits EPAPmin and EPAPmax aresettings of the RPT device 4000, set for example by hard-coding duringconfiguration of the RPT device 4000 or by manual entry through theinput device 4220.

There are interdependencies between the pressure support limits Amax andAmin, the EPAP limits EPAPmin and EPAPmax, and Plimit, where Plimit isan overall maximum pressure deliverable by the RPT device 4000. Theseinterdependencies may be expressed asEPAPMin+AMax≤PlimitEPAPMax+Amin≤Plimit

During auto-EPAP ventilation therapy, pressure support A may be reduced(no lower than Amin) to allow the EPAP to increase to stabilise theupper airway. In other words, first priority is given to maintaining astable upper airway with an appropriate EPAP, then to providingventilation therapy.

The method 8000 starts at step 8010, which computes an increaseShapeRxIncrease to the EPAP based on abnormal inspiratory waveformshape, specifically the extent of inspiratory flow limitation determinedby the inspiratory flow limitation determination algorithm 4324 andM-shape determined by the M-shape detection algorithm 4326. Step 8010 isreferred to as the “Shape Doctor” as its output ShapeRxIncrease is a“prescription” based on the shape of the inspiratory portion of therespiratory flow rate waveform. Step 8010 is described in more detailbelow with reference to FIG. 8B.

The next step 8020 computes an increase ApneaRxIncrease to the EPAPbased on one or more recent apneas detected by the apnea detectionalgorithm 4325. Step 8020 is referred to as the “Apnea Doctor” as itsoutput ApneaRxIncrease is a “prescription” based on the severity of theone or more recent apneas. Step 8020 is described in more detail belowwith reference to FIG. 8E.

Step 8030 follows, at which the EPAP auto-titration method 8000 updatesthe desired EPAP using the current EPAP value and the increasesShapeRxIncrease and ApneaRxIncrease prescribed by the Shape Doctor (step8010) and the Apnea Doctor (step 8020). Step 8030 is described in moredetail below with reference to FIG. 8G.

The final step 8040 slews the current EPAP to the desired EPAP at a slewrate of either 1 cmH₂O per second (for increases in the current EPAP) or−1 cmH₂O (for decreases in the current EPAP). In one implementation, thecurrent EPAP is not increased during inspiration (i.e. when the phase isbetween 0 and 0.5), but it may be decreased during inspiration.

7.6.2.1.1 Shape Doctor

FIG. 8B is a flow chart illustrating a method 8100 that may be used toimplement step 8010 (the “Shape Doctor”) of the method 8000.

The method 8100 starts at step 8105, which checks whether the expectednormal ventilation Vnorm is less than a threshold, in one implementationset to 1 litre/minute. As mentioned above, the expected normalventilation Vnorm may be the typical recent ventilation Vtyp returned bythe typical recent ventilation determination algorithm 4328. If so(“Y”), step 8110 sets ShapeRxIncrease to zero, because such a low valueindicates some kind of error condition, and it would be unwise toproceed with calculation of an EPAP increase.

Otherwise (“N”), step 8115 computes the current inspiratory andexpiratory tidal volumes Vi and Ve from the respiratory flow rate signalQr. Step 8120 averages the two tidal volumes Vi and Ve to obtain theaverage tidal volume Vt. The next step 8125 computes the “breathwiseventilation” (in litres/min) by multiplying the average tidal volume Vtby sixty seconds and dividing by the duration Ttot of the currentbreath. Step 8130 then divides the breathwise ventilation by theexpected normal ventilation Vnorm to obtain a (unitless) value for therelative ventilation (relative Ventilation). If relative Ventilation issignificantly less than one, the current breath is small in relation tothe expected normal ventilation Vnorm, indicating significant breathwisehypoventilation. Step 8135 formalises this relation by computing avariable significantBreathHypoventilation that decreases from 1 to 0 asrelative Ventilation increases above a hypoventilation threshold. In oneimplementation, step 8135 computes significantBreathHypoventilation asfollows:

significantBreathHypoventilation = FuzzyDeweight(         relativeVentilation,        LOWER_HYPOVENT_THRESH, 1.0,       UPPER_HYPOVENT_THRESH, 0.0              )

where LOWER_HYPOVENT_THRESH and UPPER_HYPOVENT_THRESH are constantthresholds, equal in one implementation to 0.2 and 0.8 respectively.

Conversely, if relative Ventilation is significantly greater than one,the current breath is large in relation to the expected normalventilation Vnorm, indicating significant breathwise hyperventilation.

At the next step 8140, the method 8100 computes a variablerecentBreathFlowLimitedHypovent from the current and recent values ofsignificantBreathHypoventilation. The general effect of step 8140 isthat recentBreathFlowLimitedHypovent is higher when there has been arecent persistence of significant breathwise hypoventilation inconjunction with flow-limited or M-shaped inspiratory waveforms overmultiple breaths, rather than just a single-breath instance thereof.Step 8140 will be described in more detail below with reference to FIG.8C.

The method 8000 continues at step 8145, which computes a fuzzy truthvariable mShapeRxProportion reflecting the (real-valued) M-shapeindicator variable RxProportion returned by the M-shape detectionalgorithm 4326. In one implementation, step 8145 takes the maximum ofRxProportion and 0, in other words puts a floor of 0 under the value ofRxProportion. Step 8150 then computes a fuzzy truth variableflowLimitedValue as the “fuzzy OR” of the fuzzy truth variablesmShapeRxProportion (indicating the degree of M-shape) and flowLimitation(indicating the degree of flow limitation).

The next step 8155 computes a flow limitation thresholdflowLimitedThreshold with which flowLimitedValue is to be compared. Theflow limitation threshold flowLimitedThreshold is computed from thecurrent value of EPAP in such a way that the flow limitation thresholdflowLimitedThreshold increases as the current value of EPAP increases.By this means, the degree of abnormality of inspiratory waveform shaperequired for the Shape Doctor to prescribe an increase of EPAP increasesas EPAP itself increases. In one implementation, step 8155 computesflowLimitedThreshold as follows:

flowLimitedThreshold = FuzzyDeweight(           EPAP,      LOWER_EPAP_THRESH, 0.0,       MIDDLE_EPAP_THRESH, 0.3,      UPPER_EPAP_THRESH, 0.8             )

where LOWER_EPAP_THRESH, MIDDLE_EPAP_THRESH, and UPPER_EPAP_THRESH areconstant thresholds, equal in one implementation to 14 cmH₂O, 18 cmH₂O,and 20 cmH₂O respectively.

The Shape Doctor's prescribed EPAP increase, ShapeRxIncrease, is setproportional to the amount (if any) by which between flowLimitedValueexceeds flowLimitedThreshold. The increase may be set as a function ofone or more multipliers, such as a dynamically computed multiplier.Thus, the method 8100 may first compute any one or more of the followingthree multipliers on the constant of proportionality, and which maycollectively be considered a multiplier. For example, step 8160 computesa base pressure-related multiplier flowLimitedRxMultiplier such thatflowLimitedRxMultiplier, and hence the final prescriptionShapeRxIncrease, generally decreases as the current EPAP increases. Inone implementation, flowLimitedRxMultiplier is computed as follows:

flowLimitedRxMultiplier = FuzzyDeweight(             EPAP,        LOWER_MULT1_THRESH, 1.0,         MIDDLE_MULT1_THRESH, 0.7,        MIDDLE2_MULT1_THRESH, 0.4,         UPPER_MULT1_THRESH, 0.0,              )

where LOWER_MULTI_THRESH, MIDDLE_MULTI_THRESH, MIDDLE2_MULTI_THRESH, andUPPER_MULTI_THRESH are constant thresholds, equal in one implementationto 10 cmH₂O, 15 cmH₂O, 19 cmH₂O, and 20 cmH₂O respectively.

Step 8165 computes a leak-related multiplier leakRxMultiplier such thatleakRxMultiplier, and hence the prescription ShapeRxIncrease, generallydecreases as the leak flow rate estimate Ql (from the leak flow rateestimation algorithm 4316) increases. In one implementation,leakRxMultiplier is computed as follows:

leakRxMultiplier = FuzzyDeweight(           Ql,       LOWER_LEAK_THRESH,1.0,       UPPER_LEAK_THRESH, 0.0           )

where LOWER_LEAK_THRESH and UPPER_LEAK_THRESH are constant thresholds,set in one implementation to 30 litres/minute and 60 litres/minuterespectively.

Step 8170 then computes a ventilation-related multiplierflowLimRxPropRelVent based on the current breathwise ventilationrelative to the expected normal ventilation Vnorm (expressed in relativeVentilation, computed at step 8130), the amount of flow limitation orM-shape (expressed in flowLimitedValue, computed at step 8150), and/orthe amount of recent persistent flow-limited significant breathwisehypoventilation (expressed in recentBreathFlowLimitedHypovent, computedat step 8140). Step 8170 is described in more detail below withreference to FIG. 8D.

The final step 8175 computes the Shape Doctor's prescriptionShapeRxIncrease by first checking whether flowLimitedValue exceedsflowLimitedThreshold. If not, ShapeRxIncrease is set to zero. Otherwise,step 8175 computes ShapeRxIncrease as follows:ShapeRxIncrease=(flowLimitedValue−flowLimitedThreshold)*flowLimitedRxMultiplier*leakRxMultiplier*flowLim RxPropRelVent*EPAP_GAIN

where EPAP_GAIN is a constant. In one implementation, EPAP_GAIN is setto 0.2 cmH₂O.

FIG. 8C is a flow chart illustrating a method 8200 that may be used toimplement step 8140 of the method 8100 that computes a variable namedrecentBreathFlowLimitedHypovent. As mentioned above, the general effectof step 8140 is that recentBreathFlowLimitedHypovent is higher whenthere has been a recent persistence of flow-limited or M-shapedsignificant breathwise hypoventilation over multiple breaths, ratherthan just a single-breath instance thereof.

The method 8200 starts at step 8210, which computes a variable OHV bymultiplying the maximum of the M-shape indicator RxProportion and theflow limitation indicator flowLimitation bysignificantBreathHypoventilation, which was computed at step 8135. OHVstands for obstructive (i.e. flow-limited) hypoventilation, of which thevalue of OHV is indicative for the current breath. Step 8210 stores thevalue of OHV at a current location in a circular buffer representing asmallish number of recent breaths. In one implementation, the circularbuffer contains eight entries. Step 8220 then checks whether thecircular buffer is full. If not (“N”), the method 8200 at step 8230 setsrecentBreathFlowLimitedHypovent to zero as there is not enough storedinformation to indicate a recent persistence of flow-limited or M-shapedsignificant breathwise hypoventilation. Otherwise (“Y”), step 8240applies adjacency weighting to each entry in the circular buffer. Theadjacency weighting is a function of the entry and its predecessor entryin the circular buffer such that the adjacency weighted entry is highestwhen both entries are approximately equal and close to one. In oneimplementation, step 8240 computes the adjacency-weighted OHV asadjacencyWeightedOHV=OHV+(1−OHV)*Min(OHV,OHVPrev)

where OHVPrev is the predecessor entry of the current entry OHV in thecircular buffer. In other implementations, other functions of OHV andOHVPrev may be used at step 8240, such as an arithmetic or geometricmean.

The next step 8250 of the method 8200 sums the squared values ofadjacencyWeightedOHV over the circular buffer. In one implementation,each squared value is weighted by a weight that is highest for thecurrent entry and decreases towards zero as the entries become lessrecent. In one implementation, the weightings (working backward throughthe circular buffer) are {1, 0.95, 0.9, 0.8, 0.7, 0.55, 0.4, 0.25}.

Finally, step 8260 computes recentBreathFlowLimitedHypovent as thesquare root of the sum of the (weighted) squared values divided by thesum of the weights used at step 8250 (if used, otherwise divided by thenumber of squared values). In other words,recentBreathFlowLimitedHypovent is a root mean squared value of therecent adjacency-weighted values of a flow-limited significanthypoventilation indicator (possibly with greatest weight given to themost recent value).

FIG. 8D is a flow chart illustrating a method 8300 that may be used toimplement step 8170 of the method 8100. As mentioned above, the step8170 computes the ventilation-related multiplier flowLimRxPropRelVent onthe final prescription of the Shape Doctor based on the indicatorrelative Ventilation of the relative size of the current breath, theamount of flow limitation or M-shape (expressed in flowLimitedValue),and the indicator recentBreathFlowLimitedHypovent of the amount ofrecent persistent flow-limited breathwise hypoventilation.

The general effect of step 8170 is that flowLimRxPropRelVent generallyhas a neutral value of 1 but tends to decrease below 1 as the relativeventilation exceeds a flow limitation threshold that increases with theamount of flow limitation or M-shape. That is, the Shape Doctor'sprescription tends to be discounted if relative hyperventilation isoccurring. However, the amount of relative hyperventilation needed todiscount the Shape Doctor's prescription increases as the severity offlow limitation or M-shape increases. If the relative ventilation issignificantly less than one, flowLimRxPropRelVent increases, possiblyabove 1, thereby amplifying the Shape Doctor's prescription, in generalproportion to the indicator recentBreathFlowLimitedHypovent of a recentpersistence of flow-limited or M-shaped significant breathwisehypoventilation.

The method 8300 starts at step 8310, which checks whether the value ofrelative Ventilation (computed at step 8130) is greater than or equal toone, i.e. the breathwise ventilation is greater than or equal to theexpected normal ventilation Vnorm. If so (“Y”), the method 8300 proceedsto step 8320, which computes a variable severeFlowLimitation togenerally increase from 0 to 1 as the amount of flow limitation orM-shape, expressed in the variable flowLimitedValue, increases. In oneimplementation, step 8320 computes severeFlowLimitation as follows:

severeFlowLimitation = FuzzyDeweight(         flowLimitedValue,        LOWER_FL_THRESH, 0.0,         UPPER_FL_THRESH, 1.0             )

where LOWER_FL_THRESH and UPPER_FL_THRESH are constant thresholds, equalin one implementation to 0.7 and 0.9 respectively.

The next step 8330 computes lower and upper relative ventilationthresholds lowerRelVentThreshold and upperRelVentThreshold on relativeventilation from the value of severeFlowLimitation computed at step8320. The lower and upper relative ventilation thresholdslowerRelVentThreshold and upperRelVentThreshold are at least 1.0 andincrease generally proportionally to severeFlowLimitation. In oneimplementation, step 8330 computes the lower and upper relativeventilation thresholds lowerRelVentThreshold and upperRelVentThresholdfrom severeFlowLimitation as follows:lowerRelVentThreshold=1+0.7*severeFlowLimitationupperRelVentThreshold=1.5+0.7*severeFlowLimitation

Finally, step 8340 computes flowLimRxPropRelVent so as to generallydecrease from 1 to 0 as relative Ventilation increases in relation tothe lower and upper relative ventilation thresholdslowerRelVentThreshold and upperRelVentThreshold computed at step 8330.In one implementation, step 8340 computes flowLimRxPropRelVent asfollows:

flowLimRxPropRelVent = FuzzyDeweight(         relativeVentilation,        lowerRelVentThreshold, 1.0,         upperRelVentThreshold, 0.0            )

Returning to step 8310, if step 8310 found that relative Ventilation wasless than 1 (“N”), step 8350 checks whether the current breath is asignificant hypoventilation, by checking whether the variablesignificantBreathHypoventilation, computed at step 8135, is greater than0. (In an implementation described above,significantBreathHypoventilation is greater than zero only if relativeVentilation is less than UPPER_HYPOVENT_THRESH). If not (“N”), step 8370sets flowLimRxPropRelVent to a neutral value of 1.0. Otherwise (“Y”),step 8360 computes flowLimRxPropRelVent to generally increase inproportion to the indicator recentBreathFlowLimitedHypovent of a recentpersistence of flow-limited or M-shaped significant breathwisehypoventilation. In one implementation, step 8360 computesflowLimRxPropRelVent as follows:flowLimRxPropRelVent=0.5+2*recentBreathFlowLimitedHypovent

The method 8300 then concludes.

7.6.2.1.2 Apnea Doctor

FIG. 8E is a flow chart illustrating a method 8400 that may be used toimplement step 8020 (the “Apnea Doctor”) of the method 8000.

As mentioned above, the purpose of the Apnea Doctor is to compute aprescribed EPAP increase ApneaRxIncrease based on apneas detected by theapnea detection algorithm 4325.

In one implementation, the apnea detection algorithm 4325 places itsdetected apneas (each characterised by start time, end time, andeffective duration) in a single list of pending apneas as soon as theyare ended.

The method 8400 therefore starts at step 8410 by sorting the list ofpending apneas in ascending order of their start times. The next step8420 removes any duplicates (i.e. apneas with the same start and endtimes) from the sorted list of pending apneas. Step 8430 follows, whichremoves from the sorted list any apneas that have already been processedby the Apnea Doctor.

At the next step 8440, the method 8400 resolves the (possiblyoverlapping) apneas in the sorted list into non-overlapping apneas. Step8450 then processes the completed apneas in the sorted, non-overlappinglist in ascending order of start time. The processing of an apnea atstep 8450 is described in more detail below with reference to FIG. 8F.The processing of the apneas in the list at step 8450 results in aprescribed EPAP due to apneas, prescribedEPAP. Step 8460 then computes aprescribed EPAP increase due to apneas, ApneaRxIncrease, fromprescribedEPAP and the current value of EPAP. In one implementation,step 8460 computes ApneaRxIncrease as follows:ApneaRxIncrease=prescribedEPAP−EPAP−ShapeRxIncrease  (7)

In one implementation, step 8460 clips ApneaRxIncrease below to zero, sothat ApneaRxIncrease cannot be negative. In an alternativeimplementation, step 8460 computes ApneaRxIncrease as follows:ApneaRxIncrease=prescribedEPAP−EPAP  (8)

If step 8460 uses equation (7) to compute ApneaRxIncrease, the effect,when step 8030 computes the desired EPAP as described below, is toincrease the desired EPAP by the greater of ApneaRxIncrease andShapeRxIncrease. If step 8460 uses equation (8) to computeApneaRxIncrease, the effect, when step 8030 computes the desired EPAP,is to increase the desired EPAP by the sum of ApneaRxIncrease andShapeRxIncrease.

Step 8460 may also use the desired EPAP rather than the current EPAP inequations (7) and (8).

The method 8400 then concludes.

FIG. 8F is a flow chart illustrating a method 8500 of processing anapnea that may be used to implement step 8450 of the method 8400. Theoutput of the method 8500 is a prescribed EPAP increase for the apnea,singleApneaIncrease, that broadly increases with the effective durationof the apnea.

One implementation of step 8450 of the method 8400 executes the method8500 once for each apnea in the list. In this implementation, before thefirst iteration of the method 8500 to process the first apnea in thelist, step 8450 sets the prescribed EPAP due to apneas (the output ofstep 8450), prescribedEPAP, to zero. After each iteration of the method8500, step 8450 increments prescribedEPAP by the value ofsingleApneaIncrease returned by that iteration.

The method 8500 starts at step 8510, which determines whether theeffective duration of the apnea is greater than or equal to a durationthreshold, equal in one implementation to nine seconds. If not (“N”),the method 8500 ends at step 8590, which sets the prescribed EPAPincrease singleApneaIncrease for the apnea to 0. Otherwise (“Y”), themethod 8500 proceeds to step 8520, which computes a variable calledHighApneaRolloffPressure, which is the value to which the method 8500would increase the current EPAP if the apnea were infinite in effectiveduration. In one implementation, step 8520 computesHighApneaRolloffPressure as the maximum of EPAPmax+2cmH₂O, and a minimumvalue of HighApneaRolloffPressure, a constant which in oneimplementation is set to 12 cmH₂O. HighApneaRolloffPressure maytherefore be greater than the value of EPAPmax. The next step 8530computes a rate constant for the approach of the EPAP to theHighApneaRolloffPressure, so as to generally decrease asHighApneaRolloffPressure increases. In one implementation of step 8530,the rate constant k (in units of s⁻¹) is computed as follows:k=(1.333/60)*(10/HighApneaRolloffPressure)

Step 8540 follows, at which the method 8500 computes a variableEPAPIncreaseWeightingFactor so as to generally increase from 0asymptotically towards 1 with the effective duration of the apnea. Inone implementation, step 8540 computes EPAPIncreaseWeightingFactor usingthe rate constant k as follows:EPAPIncreaseWeightingFactor=1−exp(−k*effectiveDuration)

The method 8500 concludes with step 8550, which computessingleApneaIncrease as the product of EPAPIncreaseWeightingFactor andthe difference between HighApneaRolloffPressure and the current value ofprescribedEPAP:singleApneaIncrease=EPAPIncreaseWeightingFactor*(HighApneaRolloffPressure−prescribedEPAP)

FIG. 8G is a flow chart illustrating a method 8600 that may be used toimplement step 8030 of the method 8000. As mentioned above, step 8030updates the desired EPAP using the current EPAP and the EPAP increasesShapeRxIncrease and ApneaRxIncrease prescribed by the Shape Doctor (step8010) and the Apnea Doctor (step 8020) respectively.

The method 8600 starts at step 8610, which checks whetherShapeRxIncrease or ApneaRxIncrease is greater than zero. If so (“Y”),step 8620 increases the desired EPAP by the sum of ShapeRxIncrease andApneaRxIncrease. The next step 8630 then clips the increased desiredEPAP to the range [EPAPmin, EPAPmax]. This is to say, step 8630 sets thedesired EPAP to the minimum of its incremented value from step 8620 andEPAPmax, and to the maximum of its incremented value from step 8620 andEPAPmin.

If step 8610 found that neither the Shape Doctor nor the Apnea Doctorprescribed a positive increase in the EPAP (“N”), the method 8600exponentially decays the desired EPAP towards EPAPmin. First, a decayfactor decayFactor to scale the exponential decay is computed. Thiscomputation is done in one of two branches, conditioned on whether thecurrent value of EPAP exceeds EPAPmin by 4 cmH₂O or less (checked atstep 8640). If so (“Y”), the step 8650 sets decayFactor to thedifference between the current EPAP and EPAPmin. If not (“N”), step 8660computes decayFactor to rise more slowly as the difference between thecurrent EPAP and EPAPmin increases. In one implementation of step 8660,the value of decayFactor is computed as follows, where the units ofpressure are cmH₂O:decayFactor=4+0.5*((current EPAP−EPAPmin)−4)

Finally, at step 8670 which follows either step 8660 or step 8650, themethod 8600 reduces the desired EPAP by an amount that is proportionalto the value of decayFactor computed at step 8650 or step 8660:desired EPAP=desired EPAP−decayFactor*(1−exp(−timeDiff/timeConstant))

where timeDiff is the time elapsed (in seconds) since the last update ofthe desired EPAP, and timeConstant is the time constant of the decay, inseconds. In one implementation, timeConstant is 20 minutes*60.

FIG. 9A contains a graph 9000 illustrating an example of the behaviourof the EPAP auto-titration method 8000 of FIG. 8A in response toepisodes of flow limitation. The graph 9000 contains a trace 9010 oftreatment pressure Pt with fixed pressure support as indicated by thearrow 9015. The treatment pressure trace 9010 shows an increase 9025 inthe EPAP in response to a first episode 9020 of flow limitation.Following the increase 9025 in the EPAP, a second episode 9030 of flowlimitation occurs. The second episode 9030 results in a second increase9035 in the EPAP. This second increase 9035 in the EPAP successfullyresolves the flow limitation, resulting in an episode 9040 of normalbreathing, which causes a gradual decrease 9045 of the EPAP. The dashedline 9050 indicates an upper limit EPAPmax on the EPAP. The dashed line9060 indicates an overall upper limit Plimit on the treatment pressurePt. The dashed line 9070 indicates a lower limit EPAPmin on the EPAP.

FIG. 9B contains a graph 9100 illustrating an example of the behaviourof the EPAP auto-titration method 8000 of FIG. 8A in response to aclosed apnea. The graph 9100 contains an upper trace 9105 of treatmentpressure Pt, and a lower trace 9110 of respiratory flow rate Qr. Anapnea 9115 is detected, during which three probe breaths 9120 aredelivered, each of duration Ti. Analysis of the respiratory flow rate Qrby the airway patency determination algorithm 4327 determines that theapnea 9115 is closed. At the termination 9125 of the apnea, the EPAP isincreased at a fixed rate of 1 cmH₂O/sec during successive inspiratoryportions 9130 until it reaches the desired EPAP 9135.

FIG. 9C contains a graph 9200 illustrating an example of the behaviourof the airway patency determination algorithm 4327 of FIG. 7H inresponse to a mixed apnea. The graph 9200 contains a trace 9205 oftreatment pressure Pt, a trace 9210 of respiratory flow rate Qr, a trace9215 indicative of the output of the airway patency determinationalgorithm 4327, and a trace 9220 indicative of the output of the apneadetection algorithm 4325. At the instant 9225, the respiratory flow ratefalls dramatically, and the apnea detection trace 9220 rises shortlyafterward at 9230 indicating an apnea state has been detected. Thetreatment pressure trace 9205 shows the delivery of the resulting seriesof probe breaths, the first probe breath being 9208. Analysis of therespiratory flow rate Qr by the airway patency determination algorithm4327 determines that the apnea is initially closed, so the airwaypatency trace 9215 rises at the point 9235 to a value indicative of aclosed airway. The flow rate trace 9210 then at the point 9240 begins toshow a significant response to the probe breaths, for example the probebreath 9243. Analysis of the respiratory flow rate Qr by the airwaypatency determination algorithm 4327 determines that the apnea is open,so the airway patency trace 9215 falls at the point 9245 to a valueindicative of an open airway. The flow rate trace 9210 then at the point9250 begins to show no significant response to the probe breaths, forexample the probe breath 9253. Analysis of the respiratory flow rate Qrby the airway patency determination algorithm 4327 determines that theapnea is closed, so the airway patency trace 9215 returns at the point9255 to the value indicative of a closed airway.

FIG. 9D contains expansions of various sections of the graph 9200 ofFIG. 9C. The trace 9300 is an expansion of the treatment pressure trace9205 during the probe breath 9208, and the trace 9310 is an expansion ofthe respiratory flow rate trace 9210 during that probe breath 9208. Thebreath size indicator Sz for the probe breath 9208, as computed by step7730 using the respiratory flow rate trace 9310, was below the lowerthreshold computed by step 7740, and the method 7700 thereforedetermined that the airway was closed at step 7770.

The trace 9320 is an expansion of the treatment pressure trace 9205during the probe breath 9243, and the trace 9330 is an expansion of therespiratory flow rate trace 9210 during that probe breath 9243. Thearrow 9321 indicates the extent of the trimmed inspiratory waveformreturned by step 7710. The straight-line approximation 9322 to thetrimmed inspiratory waveform has y-intercept 9324 and final value 9326.The inspiratory flatness ratio, computed at step 7810 as the ratio of9326 to 9324, was found at step 7840 to be below the inspiratorythreshold, so the method 7700 determined that the airway was open atstep 7870.

The trace 9340 is an expansion of the treatment pressure trace 9205during the probe breath 9253, and the trace 9350 is an expansion of therespiratory flow rate trace 9210 during that probe breath 9253. Thearrow 9331 indicates the extent of the trimmed inspiratory waveformreturned by step 7710. The straight-line approximation 9332 to thetrimmed inspiratory waveform has y-intercept 9334 and final value 9336.The inspiratory flatness ratio, computed at step 7810 as the ratio of9336 to 9334, was found at step 7840 to be above the inspiratorythreshold, and the expiratory flatness ratio was likewise found at step7850 to be above the expiratory threshold, so the method 7700 determinedthat the airway was closed at step 7860.

7.7 Glossary

For the purposes of the present technology disclosure, in certain formsof the present technology, one or more of the following definitions mayapply. In other forms of the present technology, alternative definitionsmay apply.

7.7.1 General

Air: In certain forms of the present technology, air may be taken tomean atmospheric air, and in other forms of the present technology airmay be taken to mean some other combination of breathable gases, e.g.atmospheric air enriched with oxygen.

Ambient: In certain forms of the present technology, the term ambientwill be taken to mean (i) external of the treatment system or patient,and (ii) immediately surrounding the treatment system or patient.

For example, ambient humidity with respect to a humidifier may be thehumidity of air immediately surrounding the humidifier, e.g. thehumidity in the room where a patient is sleeping. Such ambient humiditymay be different to the humidity outside the room where a patient issleeping.

In another example, ambient pressure may be the pressure immediatelysurrounding or external to the body.

In certain forms, ambient (e.g., acoustic) noise may be considered to bethe background noise level in the room where a patient is located, otherthan for example, noise generated by an RPT device or emanating from amask or patient interface. Ambient noise may be generated by sourcesoutside the room.

Respiratory Pressure Therapy (RPT): The application of a supply of airto an entrance to the airways at a treatment pressure that is typicallypositive with respect to atmosphere.

Continuous Positive Airway Pressure (CPAP) therapy: Respiratory pressuretherapy in which the treatment pressure is approximately constantthrough a respiratory cycle of a patient. In some forms, the pressure atthe entrance to the airways will be slightly higher during expiration,and slightly lower during inspiration. In some forms, the pressure willvary between different respiratory cycles of the patient, for example,being increased in response to detection of indications of partial upperairway obstruction, and decreased in the absence of indications ofpartial upper airway obstruction.

Patient: A person, whether or not they are suffering from a respiratorydisorder.

Automatic Positive Airway Pressure (APAP) therapy: CPAP therapy in whichthe treatment pressure is automatically adjustable, e.g. from breath tobreath, between minimum and maximum limits, depending on the presence orabsence of indications of SDB events.

7.7.2 Aspects of the Respiratory Cycle

Apnea: According to some definitions, an apnea is said to have occurredwhen flow falls below a predetermined threshold for a duration, e.g. 10seconds. A closed apnea will be said to have occurred when, despitepatient effort, some obstruction of the airway does not allow air toflow. An open apnea will be said to have occurred when an apnea isdetected that is due to a reduction in breathing effort, or the absenceof breathing effort, despite the airway being open (patent). A mixedapnea occurs when a reduction or absence of breathing effort coincideswith an obstructed airway.

Breathing rate: The rate of spontaneous respiration of a patient,usually measured in breaths per minute.

Duty cycle, or inspiratory fraction: The ratio of inspiratory time, Ti,to total breath time, Ttot.

Effort (breathing): Breathing effort will be said to be the work done bya spontaneously breathing person attempting to breathe.

Expiratory portion of a breathing cycle: The period from the start ofexpiratory flow to the start of inspiratory flow.

Flow limitation: Flow limitation will be taken to be the state ofaffairs in a patient's respiration where an increase in effort by thepatient does not give rise to a corresponding increase in flow. Whereflow limitation occurs during an inspiratory portion of a breathingcycle it may be described as inspiratory flow limitation. Where flowlimitation occurs during an expiratory portion of the breathing cycle itmay be described as expiratory flow limitation.

Types of flow-limited inspiratory waveforms:

(i) (Classically) Flattened: Having a rise followed by a relatively flatportion, followed by a fall.

(ii) M-shaped: Having two local peaks, one at the early part, and one atthe late section, and a relatively flat portion between the two peaks.

(iii) Chair-shaped: Having a single local peak, the peak being at theearly part, followed by a relatively flat portion.

(iv) Reverse-chair shaped: Having a relatively flat portion followed bysingle local peak, the peak being at the late section.

Hypopnea: A reduction in flow, but not a cessation of flow. In one form,a hypopnea may be said to have occurred when there is a reduction inflow below a threshold rate for a duration. A central hypopnea may besaid to have occurred when a hypopnea is detected that is due to areduction in breathing effort.

Hyperpnea: An increase in flow to a level higher than normal flow rate.

Hypoventilation: Hypoventilation is said to occur when the amount of gasexchange taking place over some timescale is below the currentrequirements of the patient.

Hyperventilation: Hyperventilation is said to occur when the amount ofgas exchange taking place over some timescale is above the currentrequirements of the patient.

Inspiratory portion of a breathing cycle: The period from the start ofinspiratory flow to the start of expiratory flow will be taken to be theinspiratory portion of a breathing cycle.

Patency (airway): The degree of the airway being open, or the extent towhich the airway is open. A patent airway is open. Airway patency may bequantified, for example with a value of one (1) being patent, and avalue of zero (0), being closed (obstructed).

Positive End-Expiratory Pressure (PEEP): The pressure above atmospherein the lungs that exists at the end of expiration.

Peak flow rate (Qpeak): The maximum value of flow rate during theinspiratory portion of the respiratory flow rate waveform.

Respiratory flow rate, airflow rate, patient airflow rate, respiratoryairflow rate (Qr): These synonymous terms may be understood to refer tothe RPT device's estimate of respiratory airflow rate, as opposed to“true respiratory flow” or “true respiratory airflow”, which is theactual respiratory flow rate experienced by the patient, usuallyexpressed in litres per minute.

Tidal volume (Vt): The volume of air inspired or expired per breathduring normal breathing, when extra effort is not applied. This quantitymay be more specifically defined as inspiratory tidal volume (Vi) orexpiratory tidal volume (Ve).

Inspiratory Time (Ti): The duration of the inspiratory portion of therespiratory flow rate waveform.

Expiratory Time (Te): The duration of the expiratory portion of therespiratory flow rate waveform.

Total (breath) Time (Ttot): The total duration between the start of theinspiratory portion of one respiratory flow rate waveform and the startof the inspiratory portion of the following respiratory flow ratewaveform.

Typical recent ventilation: The value of ventilation around which recentvalues over some predetermined timescale tend to cluster, that is, ameasure of the central tendency of the recent values of ventilation.

Upper airway obstruction (UAO): includes both partial and total upperairway obstruction. This may be associated with a state of flowlimitation, in which the level of flow increases only slightly or mayeven decrease as the pressure difference across the upper airwayincreases (Starling resistor behaviour).

Ventilation (Vent): A measure of the total amount of gas being exchangedby the patient's respiratory system. Measures of ventilation may includeone or both of inspiratory and expiratory flow, per unit time. Whenexpressed as a volume per minute, this quantity is often referred to as“minute ventilation”. Minute ventilation is sometimes given simply as avolume, understood to be the volume per minute.

7.7.3 RPT Device Parameters

Flow rate: The instantaneous volume (or mass) of air delivered per unittime. While flow rate and ventilation have the same dimensions of volumeor mass per unit time, flow rate is measured over a much shorter periodof time. In some cases, a reference to flow rate will be a reference toa scalar quantity, namely a quantity having magnitude only. In othercases, a reference to flow rate will be a reference to a vectorquantity, namely a quantity having both magnitude and direction. Whereit is referred to as a signed quantity, a flow rate may be nominallypositive for the inspiratory portion of a breathing cycle of a patient,and hence negative for the expiratory portion of the breathing cycle ofa patient. Flow rate will be given the symbol Q. ‘Flow rate’ issometimes shortened to simply ‘flow’. Total flow rate, Qt, is the flowrate of air leaving the RPT device. Vent flow rate, Qv, is the flow rateof air leaving a vent to allow washout of expired gases. Leak flow rate,Ql, is the flow rate of leak from a patient interface system.Respiratory flow rate, Qr, is the flow rate of air that is received intothe patient's respiratory system.

Leak: The word leak will be taken to be an unintended flow of air. Inone example, leak may occur as the result of an incomplete seal betweena mask and a patient's face. In another example leak may occur in aswivel elbow to the ambient.

Pressure: Force per unit area. Pressure may be measured in a range ofunits, including cmH₂O (centimetres of water), g-f/cm², and hectopascals(hPa). 1 cmH₂O is equal to 1 g-f/cm² and is approximately 0.98 hPa. Inthis specification, unless otherwise stated, pressure is given in unitsof cmH₂O. The pressure in the patient interface is given the symbol Pm,while the treatment pressure, which represents a target value to beachieved by the mask pressure Pm at the current instant of time, isgiven the symbol Pt.

7.7.4 Terms for Ventilators

Backup rate: A parameter of a ventilator that establishes the minimumbreathing rate (typically in number of breaths per minute) that theventilator will deliver to the patient, if not triggered by spontaneousrespiratory effort.

Cycling: The termination of a ventilator's inspiratory phase. When aventilator delivers a breath to a spontaneously breathing patient, atthe end of the inspiratory portion of the breathing cycle, theventilator is said to be cycled to stop delivering the breath.

Expiratory positive airway pressure (EPAP): a base pressure, to which apressure varying within the breath is added to produce the desired maskpressure which the ventilator will attempt to achieve at a given time.

End expiratory pressure (EEP): Desired mask pressure which theventilator will attempt to achieve at the end of the expiratory portionof the breath. If the pressure waveform template Π(Φ) is zero-valued atthe end of expiration, i.e. Π(Φ)=0 when Φ=1, the EEP is equal to theEPAP.

Inspiratory positive airway pressure (IPAP): Maximum desired maskpressure which the ventilator will attempt to achieve during theinspiratory portion of the breath.

Pressure support: A number that is indicative of the increase inpressure during ventilator inspiration over that during ventilatorexpiration, and generally means the difference in pressure between themaximum value during inspiration and the base pressure (e.g.,PS=IPAP−EPAP). In some contexts pressure support means the differencewhich the ventilator aims to achieve, rather than what it actuallyachieves.

Servo-ventilator: A ventilator that measures patient ventilation, has atarget ventilation, and which adjusts the level of pressure support tobring the patient ventilation towards the target ventilation.

Spontaneous/Timed (S/T): A mode of a ventilator or other device thatattempts to detect the initiation of a breath of a spontaneouslybreathing patient. If however, the device is unable to detect a breathwithin a predetermined period of time, the device will automaticallyinitiate delivery of the breath.

Swing: Equivalent term to pressure support.

Triggering: When a ventilator delivers a breath of air to aspontaneously breathing patient, it is said to be triggered to do so atthe initiation of the respiratory portion of the breathing cycle by thepatient's efforts.

Ventilator: A mechanical device that provides pressure support to apatient to perform some or all of the work of breathing.

7.7.5 Anatomy of the Respiratory System

Diaphragm: A sheet of muscle that extends across the bottom of the ribcage. The diaphragm separates the thoracic cavity, containing the heart,lungs and ribs, from the abdominal cavity. As the diaphragm contractsthe volume of the thoracic cavity increases and air is drawn into thelungs.

Larynx: The larynx, or voice box houses the vocal folds and connects theinferior part of the pharynx (hypopharynx) with the trachea.

Lungs: The organs of respiration in humans. The conducting zone of thelungs contains the trachea, the bronchi, the bronchioles, and theterminal bronchioles. The respiratory zone contains the respiratorybronchioles, the alveolar ducts, and the alveoli.

Nasal cavity: The nasal cavity (or nasal fossa) is a large air filledspace above and behind the nose in the middle of the face. The nasalcavity is divided in two by a vertical fin called the nasal septum. Onthe sides of the nasal cavity are three horizontal outgrowths callednasal conchae (singular “concha”) or turbinates. To the front of thenasal cavity is the nose, while the back blends, via the choanae, intothe nasopharynx.

Pharynx: The part of the throat situated immediately inferior to (below)the nasal cavity, and superior to the oesophagus and larynx. The pharynxis conventionally divided into three sections: the nasopharynx(epipharynx) (the nasal part of the pharynx), the oropharynx(mesopharynx) (the oral part of the pharynx), and the laryngopharynx(hypopharynx).

7.7.6 Mathematical Terms

Fuzzy logic is used in a number of places in this disclosure. Thefollowing is used to indicate a fuzzy membership function, which outputsa “fuzzy truth variable” in the range [0, 1], 0 representing “fuzzyfalse” and 1 representing “fuzzy true”:FuzzyMember(ActualQuantity,ReferenceQuantity1,FuzzyTruthValueAtReferenceQuantity1,ReferenceQuantity2,FuzzyTruthValueAtReferenceQuantity2,. . . ,ReferenceQuantityN,FuzzyTruthValueAtReferenceQuantityN)

A fuzzy membership function is defined as

${{FuzzyMember}\left( {x,x_{1},f_{1},x_{2},f_{2},\ldots\mspace{11mu},x_{N},f_{N}} \right)} = \left\{ {{\begin{matrix}{f_{1},{x < x_{1}}} \\{f_{N},{x \geq x_{N}}} \\{{{InterpOnInterval}\left( {x,x_{k},f_{k},x_{k + 1},f_{k + 1}} \right)},{x_{k} \leq x < x_{k + t}},{1 \leq k \leq N}}\end{matrix}{where}{{InterpOnInterval}\left( {x,x_{k},f_{k},x_{k + 1},f_{k + 1}} \right)}} = \left\{ {\begin{matrix}{{f_{k} + \frac{\left( {f_{k + 1} - f_{k}} \right)\left( {x - x_{k}} \right)}{x_{k + 1} - x_{k}}},} & {x_{k} \neq x_{k + 1}} \\f_{k} & {otherwise}\end{matrix},} \right.} \right.$the f_(j) are fuzzy truth variables, and x and the x_(j) are realnumbers.

The function “FuzzyDeweight” is defined in the same way as“FuzzyMember”, except that the values f_(k) are interpreted as realnumbers rather than fuzzy truth variables, and the output is also a realnumber.

The “fuzzy OR” of fuzzy truth variables is the maximum of those values;the “fuzzy AND” of fuzzy truth variables is the minimum of these values.These operations on two or more fuzzy truth variables will be indicatedby the names FuzzyOr and FuzzyAnd. It is to be understood that othertypical definitions of these fuzzy operations would work similarly inthe present technology.

Exponential decay towards zero with a time that during any period ofdecay starting at time t=T, the value of the decaying quantity V isgiven by

${V(t)} = {{V(T)}*{\exp\left( {- \frac{t - T}{\tau}} \right)}}$

Exponential decay is sometimes parametrised by a rate constant k ratherthan a time constant τ. A rate constant k gives the same decay functionas a time constant τ if k=1/τ.

The inner product of two functions ƒ and g on some interval I is definedas

$\left\langle {f,g} \right\rangle_{I} = {\int\limits_{I}{{f(t)}{g(t)}{dt}}}$

7.8 Other Remarks

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in Patent Office patent files orrecords, but otherwise reserves all copyright rights whatsoever.

Unless the context clearly dictates otherwise and where a range ofvalues is provided, it is understood that each intervening value, to thetenth of the unit of the lower limit, between the upper and lower limitof that range, and any other stated or intervening value in that statedrange is encompassed within the technology. The upper and lower limitsof these intervening ranges, which may be independently included in theintervening ranges, are also encompassed within the technology, subjectto any specifically excluded limit in the stated range. Where the statedrange includes one or both of the limits, ranges excluding either orboth of those included limits are also included in the technology.

Furthermore, where a value or values are stated herein as beingimplemented as part of the technology, it is understood that such valuesmay be approximated, unless otherwise stated, and such values may beutilized to any suitable significant digit to the extent that apractical technical implementation may permit or require it.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this technology belongs. Although any methods andmaterials similar or equivalent to those described herein can also beused in the practice or testing of the present technology, a limitednumber of the exemplary methods and materials are described herein.

When a particular material is identified as being used to construct acomponent, obvious alternative materials with similar properties may beused as a substitute. Furthermore, unless specified to the contrary, anyand all components herein described are understood to be capable ofbeing manufactured and, as such, may be manufactured together orseparately.

It must be noted that as used herein and in the appended claims, thesingular forms “a”, “an”, and “the” include their plural equivalents,unless the context clearly dictates otherwise.

All publications mentioned herein are incorporated herein by referencein their entirety to disclose and describe the methods and/or materialswhich are the subject of those publications. The publications discussedherein are provided solely for their disclosure prior to the filing dateof the present application. Nothing herein is to be construed as anadmission that the present technology is not entitled to antedate suchpublication by virtue of prior invention. Further, the dates ofpublication provided may be different from the actual publication dates,which may need to be independently confirmed.

The terms “comprises” and “comprising” should be interpreted asreferring to elements, components, or steps in a non-exclusive manner,indicating that the referenced elements, components, or steps may bepresent, or utilized, or combined with other elements, components, orsteps that are not expressly referenced.

The subject headings used in the detailed description are included onlyfor the ease of reference of the reader and should not be used to limitthe subject matter found throughout the disclosure or the claims. Thesubject headings should not be used in construing the scope of theclaims or the claim limitations.

Although the technology herein has been described with reference toparticular examples, it is to be understood that these examples aremerely illustrative of the principles and applications of thetechnology. In some instances, the terminology and symbols may implyspecific details that are not required to practice the technology. Forexample, although the terms “first” and “second” (etc.) may be used,unless otherwise specified, they are not intended to indicate any orderbut may be utilised to distinguish between distinct elements.Furthermore, although process steps in the methodologies may bedescribed or illustrated in an order, such an ordering is not required.Those skilled in the art will recognize that such ordering may bemodified and/or aspects thereof may be conducted concurrently or evensynchronously.

It is therefore to be understood that numerous modifications may be madeto the illustrative examples and that other arrangements may be devisedwithout departing from the spirit and scope of the technology.

7.9 Reference Label List

patient 1000 patient interface 3000 non-invasive patient interface 3000seal-forming structure 3100 plenum chamber 3200 structure 3300 vent 3400connection port 3600 forehead support 3700 RPT device 4000 externalhousing 4010 upper portion 4012 portion 4014 panel 4015 chassis 4016handle 4018 pneumatic block 4020 pneumatic components 4100 air filter4110 inlet air filter 4112 outlet air filter 4114 inlet muffler 4122outlet muffler 4124 pressure generator 4140 controllable blower 4142motor 4144 air circuit 4170 supplemental oxygen 4180 electricalcomponents 4200 Printed Circuit Board Assembly 4202 electrical powersupply 4210 input device 4220 central controller 4230 clock 4232 therapydevice controller 4240 protection circuits 4250 memory 4260 transducers4270 pressure sensor 4272 flow rate sensor 4274 motor speed transducer4276 data communication interface 4280 remote external communicationnetwork 4282 local external communication network 4284 remote externaldevice 4286 local external device 4288 output devices 4290 displaydriver 4292 display 4294 algorithms 4300 pre-processing module 4310pressure compensation algorithm 4312 vent flow rate estimation 4314 leakflow rate estimation algorithm 4316 respiratory flow rate estimationalgorithm 4318 therapy engine module 4320 phase determination algorithm4321 waveform determination algorithm 4322 ventilation determinationalgorithm 4323 inspiratory flow limitation detection 4324 apneadetection algorithm 4325 M-shape detection algorithm 4326 airway patencydetermination algorithm 4327 typical recent ventilation determination4328 therapy parameter determination 4329 algorithm therapy controlmodule 4330 humidifier 5000 humidifier inlet 5002 humidifier outlet 5004humidifier base 5006 humidifier reservoir 5110 humidifier reservoir dock5130 heating element 5240 humidifier controller 5250 method 7000 step7010 step 7020 step 7030 step 7040 method 7100 step 7110 step 7120 step7130 step 7140 step 7150 step 7160 step 7170 step 7180 method 7200 step7210 step 7220 step 7230 step 7240 step 7250 step 7260 method 7300 step7310 step 7320 step 7330 step 7340 step 7350 step 7360 step 7370 step7375 step 7380 step 7390 method 7400 step 7410 step 7420 step 7440 step7450 step 7455 step 7460 step 7465 step 7470 step 7475 method 7500 step7510 step 7520 step 7530 step 7535 step 7540 step 7550 step 7560 step7570 step 7580 method 7600 step 7610 steps 7615 steps 7620 step 7630step 7635 step 7640 step 7645 step 7650 step 7655 step 7660 step 7665step 7670 step 7680 step 7685 step 7690 method 7700 step 7710 step 7720step 7730 step 7740 step 7750 step 7760 step 7770 step 7780 step 7790step 7810 step 7820 step 7830 step 7840 step 7850 step 7860 step 7870method 8000 step 8010 step 8020 step 8030 step 8040 method 8100 step8105 step 8110 step 8115 step 8120 step 8125 step 8130 step 8135 step8140 step 8145 step 8150 step 8155 step 8160 step 8165 step 8170 step8175 method 8200 step 8210 step 8220 step 8230 step 8240 step 8250 step8260 method 8300 step 8310 step 8320 step 8330 step 8340 step 8350 step8360 step 8370 method 8400 step 8410 step 8420 step 8430 step 8440 step8450 step 8460 method 8500 step 8510 step 8520 step 8530 step 8540 step8550 step 8590 method 8600 step 8610 step 8620 step 8630 step 8640 step8650 step 8660 step 8670 graph 9000 treatment pressure trace 9010 arrow9015 first episode 9020 increase 9025 second episode 9030 secondincrease 9035 episode 9040 gradual decrease 9045 dashed line 9050 dashedline 9060 dashed line 9070 graph 9100 upper trace 9105 trace 9110 apnea9115 breaths 9120 termination 9125 inspiratory portions 9130 EPAP 9135graph 9200 treatment pressure trace 9205 probe breath 9208 flow ratetrace 9210 airway patency trace 9215 apnea detection trace 9220 instant9225 point 9235 point 9240 probe breath 9243 point 9245 point 9250 probebreath 9253 point 9255 trace 9300 respiratory flow rate trace 9310 trace9320 arrow 9321 straight-line approximation 9322 y-intercept 9324 finalvalue 9326 trace 9330 arrow 9331 straight-line approximation 9332y-intercept 9334 final value 9336 trace 9340 trace 9350

The invention claimed is:
 1. Apparatus for treating a respiratorydisorder in a patient, comprising: a pressure generator configured todeliver a flow of air at positive pressure to an airway of the patientthrough a patient interface; a sensor configured to generate a signalrepresentative of respiratory flow rate of the patient; and a controllerconfigured to: control the pressure generator to deliver ventilationtherapy having a base pressure and a pressure support through thepatient interface; detect an apnea from the signal representative ofrespiratory flow rate of the patient; control the pressure generator todeliver one or more probe breaths to the patient during the apnea;determine patency of the patient's airway from a waveform of therespiratory flow rate signal in response to one of the one or more probebreaths; compute an effective duration of the apnea based on the patencyof the airway; and adjust a set point for the base pressure of theventilation therapy in response to the apnea based on the effectiveduration of the apnea.
 2. A method of treating a respiratory disorder ina patient, the method comprising: controlling a pressure generator todeliver a ventilation therapy through a patient interface to thepatient, the ventilation therapy having a base pressure and a pressuresupport; detecting, in a controller of the pressure generator, an apneafrom a signal representative of respiratory flow rate of the patient;controlling the pressure generator to deliver one or more probe breathsto the patient during the apnea; determining patency of the patient'sairway from a waveform of the respiratory flow rate signal in responseto one of the one or more probe breaths; computing an effective durationof the apnea based on the patency of the airway; and adjusting a setpoint for the base pressure of the ventilation therapy in response tothe apnea based on the effective duration of the apnea.
 3. The method ofclaim 2, wherein if the pressure support of the ventilation therapy isabove a threshold, the determining determines the airway patency to beclosed independent of the respiratory flow rate signal.
 4. The method ofclaim 2, wherein determining the patency comprises: computing aninspiratory flatness ratio from an inspiratory portion of therespiratory flow rate waveform in response to the probe breath,determining the airway to be open if the inspiratory flatness ratio isbelow an inspiratory threshold.
 5. The method of claim 4, furthercomprising computing the inspiratory threshold based on a relative sizeindicator of the probe breath.
 6. The method of claim 5, furthercomprising computing the relative size indicator from an indicator ofthe size of the respiratory flow rate in response to the probe breath,an upper threshold, and a lower threshold.
 7. The method of claim 6,further comprising computing the indicator of the size of therespiratory flow rate in response to the probe breath as a mean of theabsolute value of the respiratory flow rate in response to the probebreath, divided by a typical recent value of the ventilation of thepatient.
 8. The method of claim 4, wherein the inspiratory flatnessratio is computed as a final value of a straight-line approximation tothe inspiratory portion divided by an initial value of the straight-lineapproximation to the inspiratory portion.
 9. The method of claim 2,wherein determining the patency comprises: computing an expiratoryflatness ratio from an expiratory portion of the respiratory flow ratewaveform in response to the probe breath, determining the airway to beopen if the expiratory flatness ratio is below an expiratory threshold.10. The method of claim 2, wherein determining the patency furthercomprises: computing an indicator of the size of the respiratory flowrate in response to the probe breath from the respiratory flow ratesignal, and determining the patency based on the computed indicator. 11.The method of claim 10, wherein determining the patency comprisesdetermining the airway to be closed if the computed indicator is below alower threshold.
 12. The method of claim 11, wherein the lower thresholdis based on the pressure support of the ventilation therapy.
 13. Themethod of claim 10, wherein determining the patency comprisesdetermining the airway to be open if the computed indicator is above anupper threshold.
 14. The method of claim 2, wherein computing theeffective duration of the apnea based on the patency comprises computinga total duration of closed portions of the apnea.
 15. A respiratorytherapy system comprising means for: delivering a ventilation therapythrough a patient interface to a patient, the ventilation therapy havinga base pressure and a pressure support; generating a signalrepresentative of respiratory flow rate of the patient; detecting anapnea from the respiratory flow rate signal; delivering one or moreprobe breaths to the patient during the apnea; determining patency ofthe patient's airway from a waveform of the respiratory flow rate signalin response to one of the one or more probe breaths; computing aneffective duration of the apnea based on the patency of the airway; andadjusting a set point for the base pressure of the ventilation therapyin response to the apnea based on the effective duration of the apnea.16. A method of determining patency of the airway during an apnea of apatient, the method comprising: controlling a pressure generator todeliver one or more probe breaths to the patient during the apnea; anddetermining patency of the airway from a signal representative ofrespiratory flow rate of the patient during the apnea, wherein thedetermining is dependent on one or both of an output of a shapeevaluation of an inspiratory portion and an output of a shape evaluationof an expiratory portion, wherein the inspiratory portion and theexpiratory portion are each determined from a waveform of therespiratory flow rate signal that is in response to at least one of theone or more probe breaths.
 17. The method of claim 16, whereindetermining the patency of the airway comprises: computing aninspiratory flatness ratio from the inspiratory portion of therespiratory flow rate waveform in response to the probe breath,determining the airway to be open if the inspiratory flatness ratio isbelow an inspiratory threshold.
 18. The method of claim 17, furthercomprising computing the inspiratory threshold based on a relative sizeindicator of the probe breath.
 19. The method of claim 18, furthercomprising computing the relative size indicator from an indicator ofthe size of the respiratory flow rate in response to the probe breath,an upper threshold, and a lower threshold.
 20. The method of claim 19,further comprising computing the indicator of the size of therespiratory flow rate in response to the probe breath as a mean of theabsolute value of the respiratory flow rate in response to the probebreath, divided by a typical recent value of ventilation of the patient.21. The method of claim 17, wherein the inspiratory flatness ratio iscomputed as a final value of a straight-line approximation to theinspiratory portion divided by an initial value of the straight-lineapproximation to the inspiratory portion.
 22. The method of claim 16,wherein determining the patency comprises: computing an expiratoryflatness ratio from the expiratory portion of the respiratory flow ratewaveform in response to the probe breath, determining the airway to beopen if the expiratory flatness ratio is below an expiratory threshold.23. The method of claim 16, wherein determining the patency furthercomprises: computing an indicator of the size of the respiratory flowrate in response to the probe breath from the respiratory flow ratesignal, and determining the patency based on the computed indicator. 24.The method of claim 23, wherein determining the patency comprisesdetermining the airway to be closed if the computed indicator is below alower threshold.
 25. The method of claim 24, wherein the lower thresholdis based on a pressure support of ventilation therapy being delivered tothe patient.
 26. The method of claim 23, wherein determining the patencycomprises determining the airway to be open if the computed indicator isabove an upper threshold.
 27. The method of claim 16, further comprisingcomputing an effective duration of the apnea based on the patency of theairway.
 28. The method of claim 27, wherein computing the effectiveduration of the apnea based on the patency comprises computing a totalduration of closed portions of the apnea.
 29. Apparatus for treating arespiratory disorder in a patient, comprising: a pressure generatorconfigured to deliver a flow of air at positive pressure to an airway ofthe patient through a patient interface; a sensor configured to generatea signal representative of respiratory flow rate of the patient; and acontroller configured to: control the pressure generator to deliverventilation therapy through the patient interface; control the pressuregenerator to deliver one or more probe breaths to the patient during anapnea of the patient; and determine patency of the airway from awaveform of the respiratory flow rate signal during the apnea, whereindetermining the patency is dependent on one or both of an output of ashape evaluation of an inspiratory portion and an output of a shapeevaluation of an expiratory portion, wherein the inspiratory portion andthe expiratory portion are each determined from the respiratory flowrate waveform that is in response to at least one of the one or moreprobe breaths.
 30. A respiratory therapy system comprising means for:delivering a flow of air at positive pressure to an airway of a patientthrough a patient interface; generating a signal representative ofrespiratory flow rate of the patient; delivering one or more probebreaths to the patient during an apnea of the patient; and determiningpatency of the airway from a waveform of the respiratory flow ratesignal during the apnea, wherein the determining is dependent on one orboth of an output of a shape evaluation of an inspiratory portion and anoutput of a shape evaluation of an expiratory portion, wherein theinspiratory portion and the expiratory portion are each determined fromthe respiratory flow rate waveform in response to at least one of theprobe breaths.